Methods of fabricating nanostructures and nanowires and devices fabricated therefrom

ABSTRACT

One-dimensional nanostructures having uniform diameters of less than approximately 200 nm. These inventive nanostructures, which we refer to as “nanowires”, include single-crystalline homostructures as well as heterostructures of at least two single-crystalline materials having different chemical compositions. Because single-crystalline materials are used to form the heterostructure, the resultant heterostructure will be single-crystalline as well. The nanowire heterostructures are generally based on a semiconducting wire wherein the doping and composition are controlled in either the longitudinal or radial directions, or in both directions, to yield a wire that comprises different materials. Examples of resulting nanowire heterostructures include a longitudinal heterostructure nanowire (LOHN) and a coaxial heterostructure nanowire (COHN).

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. provisionalapplication serial No. 60/280,676 filed on Mar. 30, 2001, incorporatedherein by reference, and from U.S. provisional application serial No.60/349,206 filed on Jan. 15, 2002, incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under ContractNo. DE-AC03-76SF00098, awarded by the Department of Energy, Grant No.DMR-0092086, awarded by the National Science Foundation, and Grant No.CTS-0103609, awarded by the National Science Foundation. The Governmenthas certain rights in this invention.

REFERENCE TO A COMPUTER PROGRAM APPENDIX

[0003] Not Applicable

BACKGROUND OF THE INVENTION

[0004] 1. Field of the Invention

[0005] The present invention pertains generally to nanostructures, andmore particularly to a substantially crystalline nanowire structurehaving a diameter along the wire axis which varies by less thanapproximately 10% over a section exhibiting the maximum change indiameter, and which has a diameter of less than approximately 200 nm.The nanowire structure can be formed as a homostructure, as aheterostructure, and as combinations thereof.

[0006] 2. Description of the Background Art

[0007] The ability to efficiently convert energy between different forms(e.g., thermal, electrical, mechanical, and optical) as illustrated inFIG. 1 creates the infrastructure of any modern economy and is one ofthe most recognizable symbols of advances in science and engineering.Optoelectronics, for example, deals with the conversion between opticaland electronic forms, which has laid the foundation for many aspects ofmodern information technology. Conversion between thermal energy andelectrical power is the hallmark of the energy economy, where evenmarginal improvements in efficiency and conversion methods can haveenormous impact on both monetary savings, energy reserves, and theenvironment. Similarly, electromechanical energy conversion lies at theheart of many modern machines and sensors, which have found widespreaduse in technology. Given its importance, it is natural to ask whethernanoscale science and engineering can play any role in energyconversion. Clearly, in view of the continuing quest for miniaturizationand increased efficiency of devices, nanoscale devices can play a rolein energy conversion. Accordingly, there is a need for a broad spectrumof high performance energy conversion devices based on one-dimensionalinorganic nanostructures or nanowires. The present invention satisfiesthat need, as well as others, and overcomes deficiencies inherent inconventional devices.

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention pertains generally to nanostructures whichare substantially crystalline, and more particularly to one-dimensionalnanostructures having a diameter along the longitudinal axis which doesnot vary by more than approximately 10% over the section exhibiting themaximum change in diameter, and having a diameter of less thanapproximately 200 nm at the point of maximum diameter. These inventivenanostructures, which we refer to as “nanowires”, preferably comprisesubstantially monocrystalline homostructures, as well asheterostructures of at least one substantially crystalline material andone other material where an interface or junction is formedtherebetween. Heterostructures according to the present invention canalso include combinations of homostructures and heterostructures. In theevent that substantially crystalline materials are used to form theheterostructure, the resultant heterostructure will be substantiallycrystalline as well. Additionally, nanowires according to the inventioncan have various cross-sectional shapes, including, but not limited, tocircular, square, rectangular and hexagonal.

[0009] Heterostructures can be formed with any number of segments, bothlongitudinally and coaxially, where adjacent segments are substantiallycrystalline or where a substantially crystalline segment is adjacent toa material which is not substantially crystalline. Many of the nanowireheterostructures according to the present invention are generally basedon a semiconducting wire wherein the doping and composition arecontrolled in either the longitudinal or radial directions, or in bothdirections, to yield a wire that comprises different materials. Segmentsof heterostructures can be various materials, including, for example,semiconductor materials which are doped or intrinsic and arranged toform a variety of semiconductor devices with junctions such as pn, pnp,npn, pin, pip and so forth.

[0010] By way of further example, according to an aspect of theinvention, the nanowire could comprise different materials when viewedlongitudinally, such as would be the case with alternating or periodicsegments of different materials or multi-segmented nanowires where atleast two of the segments comprise different materials. We refer to thisconfiguration as a longitudinal heterostructure nanowire (LOHN). Anexample would be a LOHN where adjacent segments have different chemicalcompositions such as Si and SiGe.

[0011] According to another aspect of the invention, the nanowire wouldbe a coaxial-type structure, comprising a core of a first materialsurrounded by a jacket of a second material. We refer to thisconfiguration as a coaxial heterostructure nanowire (COHN).

[0012] The junctions between the compositionally substantiallycrystalline materials defining nanowire heterostructures according tothe present invention typically exhibit a high degree of sharpness. Forexample, in accordance with the present invention, the interface betweenthese materials can be made as sharp as approximately one atomic layerto approximately 20 nm. However, since heterostructures according to thepresent invention can comprise multiple segments either longitudinally,coaxially, or both, it is also possible to form heterostructures wheresome junctions exhibit a high degree of sharpness and others do notdepending upon the particular application and need. Furthermore, notonly can the composition of the materials forming adjacent segments besharp or gradual, but by controlling the doping of materials formingsegments of the heterostructure, it is possible to have sharp or gradualdopant transition between segments.

[0013] In certain embodiments of the present invention, thenanostructures of this invention expressly exclude structures comprisingcarbon nanotubes and/or structures comprising what are commonly referredto as “whiskers” or “nano-whiskers”.

[0014] It will be appreciated that various configurations can beachieved using the foregoing inventive structures, some of which havebeen previously described. By way of further example, and not oflimitation, these configurations can include single and multiplejunction LOHNs, single and multiple junction COHNs, combinations of LOHNand COHN structures, two-terminal configurations, N>2 terminalconfigurations, combinations of heterostructures and homostructures,homostructures with one or more electrodes (which would also be anoverall heterostructure), heterostructures with one or more electrodes,homostructures with insulators, heterostructures with insulators, andthe like. It will also be appreciated that the interface between ananowire and a terminal constitutes a heterojunction. A variety ofdevices can be fabricated using these structures and configurations,including, but not limited to, phonon bandgap devices, quantum dots thatconfine electrons in specific areas, thermoelectric devices (e.g., solidstate refrigerators and engines), photonic devices (e.g., nanolasers),nanoelectromechanical (MEM) devices (electromechanical actuators andsensors), energy conversion devices of various forms including forexample, light to mechanical energy or thermal energy to light, andother devices.

[0015] According to another aspect of the invention, a process forfabricating nanowires has been developed. In particular, this aspect ofthe invention includes a process for making a population of nanowireheterostructures with a substantially monodisperse distribution ofdiameters if the distribution of diameters within the population is lessthan or equal to approximately 50% rms, more preferably less than orequal to 20% rms, and most preferably less than 10% rms. A furtheraspect of the invention comprises a process for forming populations ofnanowires with a substantially monodisperse distribution of lengths. Apopulation of nanowires is considered to have a monodispersedistribution of lengths in the distribution of lengths within thepopulation is less than or equal to 20% rms, more preferably less thanor equal to 10% rms, more preferably less than or equal to 5% rms, andmost preferably less than 1%. A further aspect of the inventioncomprises a design for nanowires that permits batch fabrication in largequantities. Another aspect of the invention includes a laser device thatcan be formed from either a heterostructure or a homogeneous structure.

[0016] Further objects and advantages of the invention will be broughtout in the following portions of the specification, wherein the detaileddescription is for the purpose of fully disclosing preferred embodimentsof the invention without placing limitations thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

[0018]FIG. 1 is diagram illustrating conversion between different formsof energy that are enabled by 1-D semiconducting and dielectricnanowires according to the present invention.

[0019]FIG. 2 is a schematic perspective view of a coaxialheterostructure nanowire (COHN) according to the present inventionhaving a sheath over a homostructure core.

[0020]FIG. 3 is a schematic perspective view of a longitudinalheterostructure nanowire (LOHN) according to the present inventionhaving five segments (e.g., superlattice).

[0021]FIG. 4 is a schematic perspective view of a coaxialheterostructure (COHN) according to the present invention having asegmented sheath over a homostructure core.

[0022]FIG. 5 is a schematic perspective view of a coaxialheterostructure (COHN) according to the present invention having asegmented core (e.g., LOHN).

[0023]FIG. 6 is a schematic perspective view of a coaxialheterostructure (COHN) according to the present invention having asegmented core (e.g., LOHN) and a segmented sheath.

[0024]FIG. 7 is a schematic perspective view of a coaxialheterostructure (COHN) according to the present invention having asuperlattice core (e.g., LOHN).

[0025]FIG. 8 is a schematic perspective view of a coaxialheterostructure (COHN) according to the present invention having apartial sheath on a homostructure core (e.g., LOHN).

[0026]FIG. 9 is a schematic perspective view of a coaxialheterostructure (COHN) according to the present invention having apartial sheath over a segmented core (e.g., LOHN).

[0027]FIG. 10 is a schematic perspective view of a pn heterojunctionaccording to the present invention.

[0028]FIG. 11 is a schematic perspective view of a pnp, npn, pin, pipheterojunction according the present invention.

[0029]FIG. 12 is a schematic process flow diagram illustratingvapor-liquid-solid (VLS) type growth of a 1-dimensional Si nanowireaccording to the present invention using Au nanoclusters as catalystsand SiH₄ as the vapor source.

[0030]FIG. 13 is a schematic side view of a pn-type LOHN according tothe present invention using Boron doped Si as the p-type material(p-Si(B)) and Phosphorus doped Si as the n-type material (n-Si(P)).

[0031]FIG. 14 is a schematic side view of an Si/Ge LOHN according to thepresent invention.

[0032]FIG. 15 is a conduction band diagram of a coaxial heterostructurenanowire (COHN) according to the present invention.

[0033]FIG. 16 is a conduction band diagram of a longitudinalheterostructure nanowire (LOHN) according to the present invention.

[0034]FIG. 17 is a schematic of a band profile for a GaAs capped, GaSbself-assembled quantum dot according to the present invention.

[0035]FIG. 18 is a graph showing the characteristic Ballistic ElectronEmission Microscopy (BEEM) spectra for the GaSb/GaAs self-assembledquantum dot profiled in FIG. 17.

[0036]FIG. 19 is a schematic diagram of a BEEM configuration todetermine the local electronic band structure of an Si/Ge LOHN accordingto the present invention.

[0037]FIG. 20 is a graph showing the thermal conductivity of a multiwallcarbon nanotube bundle as a function of temperature, where the I°behavior suggests phonon confinement in 2-D and the monotonic increasein thermal conductivity at high temperatures indicates suppression ofphonon-phonon scattering and the presence of very long (≈1 μm) mean freepaths.

[0038]FIG. 21 is a graph showing thermopower measurements of a multiwallcarbon nanotube using a microfabricated measurement structure comprisingtwo suspended heaters that contain e-beam lithographically fabricatedwires according to the present invention, with the multiwall carbonnanotube bundle placed across the two heater sections so that the twoheater sections were bridged.

[0039]FIG. 22 is a schematic diagram of an experimental set formeasuring the mechanical motion of a piezoelectric or pyroelectricnanowire according to the present invention using an atomic forcemicroscope (AFM) cantilever probe while simultaneously measuring theelectrostatic potential across the nanowire.

[0040]FIG. 23 is a schematic side view of a vapor-liquid-solid (VLS)growth chamber for block-by-block growth of a nanowire heterostructureusing a pulsed laser.

[0041]FIG. 24 is a schematic process flow diagram illustratingvapor-liquid-solid (VLS) type growth of a 1-dimensional Si/SiGesuperlattice structure according to the present invention using thegrowth chamber shown in FIG. 23.

[0042]FIG. 25 is a schematic perspective view of an Si/SiGe superlatticenanowire array according to the present invention.

[0043]FIG. 26 is a graph showing the energy-dispersive X-rayspectroscopy (EDS) spectrum of the Ge rich region on a Si/SiGesuperlattice nanowire according to the present invention.

[0044]FIG. 27 is a graph showing the line profile of the EDS signal fromthe Si and Ge components along the growth axis of a Si/SiGe superlatticenanowire according to the present invention.

[0045]FIG. 28 is a graph showing an example of the correlation betweenthe growth rate of a nanowire according to the present invention and thediameter observed.

[0046]FIG. 29 is a graph illustrating calculated dependence of ZT on Biquantum well (2D) and quantum wire (1D) dimensions.

[0047]FIG. 30 is a schematic side view of an embodiment of athermoelectric device according to the present invention based on acomposite of n- or p-doped thermoelectric nanowire arrays embedded in apolymer matrix.

[0048]FIG. 31 is a schematic perspective view of a nanowire-polymercomposite array according to the present invention configured for lightemission.

[0049]FIG. 32 is a schematic view of an nanowire-based electron ejectionlight emitting diode/laser diode according to the present invention.

[0050]FIG. 33 is a schematic perspective view of a longitudinalheterostructure nanowire (LOHN) with quantum dots according to theinvention.

[0051]FIG. 34 is a schematic view of an embodiment of a 3-terminalnanowire device according to the present invention.

[0052]FIG. 35 is a schematic view of a second embodiment of a3-terminatl nanowire device according to the present invention.

[0053]FIG. 36 is a schematic side view of an embodiment of alongitudinally configured electromechanical transducer based onnanowires according to the present invention.

[0054]FIG. 37 is cross-section view of the transducer shown in FIG. 36taken through line 37-37.

[0055]FIG. 38 is a schematic side view of an embodiment of a transverseconfigured electromechanical transducer based on nanowires according tothe present invention.

[0056]FIG. 39 is a cross-section view of the transducer shown in FIG. 38taken through line 39-39.

[0057]FIG. 40 is a graph showing an x-ray diffraction (XRD) pattern of aZnO nanowire grown on a sapphire substrate according to the presentinvention.

[0058]FIG. 41 is a graph showing the evolution of the emission spectraresulting from increasing pump power to a ZnO nanowire on sapphireaccording to the present invention where curve a shows the spectrum atexcitation intensity below the lasing threshold, and curve b and theinset shows the spectra after the lasing threshold is exceeded.

[0059]FIG. 42 is a graph showing integrated emission intensity from aZnO nanowire on sapphire according to the present invention as afunction of optical pumping intensity.

[0060]FIG. 43 is a schematic view of a ZnO nanowire on sapphire as aresonance cavity according to the present invention with two naturallyfaceted hexagonal faces acting as reflecting mirrors.

[0061]FIG. 44 is a graph showing the decay of the luminescence from aZnO nanowire on sapphire according to the present invention using afrequency-tripled mode-locked Ti:sapphire laser for pulsed excitationand a streak camera with ps-resolution for detection.

DETAILED DESCRIPTION OF THE INVENTION

[0062] 1. Introduction.

[0063] The present invention comprises a family of nanostructures whichwe refer to as “nanowires”. Nanowires in accordance with the presentinvention generally comprise heterostructures of at least onesubstantially crystalline material and one other compositionallydifferent material where an interface or junction is formedtherebetween. However, nanowire heterostructures according to theinvention can also include heterostructures where the materials are thesame but have different crystalline orientations. Furthermore, thesurface of a nanowire according to the present invention (whether ahomostructure or a heterostructure) could be functionalized to capturespecific chemical or biological species. In the event that substantiallycrystalline materials are used to form the heterostructure, it will beappreciated that the resultant heterostructure will be substantiallycrystalline as well. Preferably at least one of the materials in theheterostructure is substantially monocrystalline. In this regard, werefer to a material as being substantially crystalline if the materialexhibits long range ordering.

[0064] A nanowire according to the present invention preferably has adiameter of less than approximately 200 nm at its maximum point, and thediameter along the longitudinal axis preferably varies by less thanapproximately 10% over the section exhibiting the maximum change indiameter. Additionally, nanowires according to the invention can havevarious cross-sectional shapes, including, but not limited, to circular,square, rectangular and hexagonal. For example, ZnO nanowires have ahexagonal cross-section, SnO₂ nanowires have a rectangularcross-section, PbSe nanowires have a square cross-section, and Si or Genanowires have a circular cross-section. In each case, the term“diameter” is intended to refer to the effective diameter, as defined bythe average of the major and minor axis of the cross-section of thestructure.

[0065] It should be appreciated that the nanowire materials of thepresent invention are fundamentally different from those commonlyreferred to as semiconductor “whiskers” formed by using basic VLS growthtechniques. It is well understood that the mechanism responsible for thegrowth of these “whiskers” is limited to the creation of semiconductorwires of a diameter greater than approximately 1 μm.

[0066] The methods of the present invention describe a method ofmodifying VLS growth in which the catalyst size is arrested to formnanowires with diameters from approximately 1 nm to approximately 200nm. Due to the quantum confinement effects, these structures arefundamentally different than whiskers, which are larger than the Bohrexciton radius of the bulk semiconductors from which they are formed,and therefore represent a unique composition of matter. The physical,optical, and electronic properties of these materials are fundamentallydifferent than would be achieved if the characteristics of whiskers weresimply extrapolated toward far smaller sizes. In the nanowire size rangethese materials represent a new form of matter, different than bulkmaterial in ways which are unique and non-obvious. The importance of thedistinction between nanowires and traditional “whiskers” should beappreciated. Whiskers operate as small “bulk” semiconductor wires andthereby provide the same functionality as wires formed by standardphotolithographic semiconductor processing techniques. The nanowiresdescribed within the present invention, however, display both electronicand optical properties that are fundamentally different than the bulkmaterial from which they are formed, and are characteristicallydifferent than that of “whiskers”.

[0067] Nanowire heterostructures according to the present inventioninclude configurations where two or more substantially monocrystallinematerials are spatially arranged in such a manner that quantumconfinement effects are exploited in new and unique ways. This approachis expected to not only open the road to scientific discoveries, butalso offer the promising prospects of dramatically changing energyconversion technology.

[0068] In certain embodiments of the invention fabrication isfacilitated with the well known vapor-liquid-solid (VLS) chemicalsynthesis process which will be described herein. The basic (unmodified)VLS process is also described in detail in the following publicationswhich are incorporated herein by reference: Wagner, R. S., “VLSMechanism of Crystal Growth”, Whisker Technology, pp. 47-119 (1970);Wagner et al., “Vapor-Liquid-Solid Mechanism of Single Crystal Growth”,Applied Physics Letters, Vol. 4., No. 5, pp. 89-90 (1964); andGivargizov, E., “Fundamental Aspects of VLS Growth”, Journal of CrystalGrowth, Vol. 31, pp. 20-30 (1975). Using the basic (unmodified) VLS, itis possible to grow monocrystalline nanowires of a wide variety ofsemiconducting materials (e.g., Si, Ge, ZnO, etc.) with an averagediameter greater than approximately 1 μm and a diameter distributiongreater than 50% and lengths up to or exceeding many millimeters. Thepresent invention provides methods of forming nanowires structures witha diameter less than approximately 200 nm and preferably in the range ofapproximately 5 nm to approximately 50 nm, with lengths in the range ofapproximately 100 nm to approximately 100 μm, preferably in the rangebetween approximately 1 μm to approximately 20 μm.

[0069] Furthermore, if the diameter of semiconducting nanowires isreduced to the range of approximately 5 nm to approximately 50 nm,quantum confinement of electrons and holes allows tailoring of theelectronic band structure of the entire nanowire or of one or moredomains within the nanowire. Such confinement can also stronglyinfluence photon and/or phonon transport in nanowires because both thephoton and/or phonon spectra and lifetimes can be significantlymodified. The importance of surface energy and growth anisotropy innanowire synthesis also affords the possibility of synthesizing phasesthat are stable in nanowire form but metastable in the bulk or as thinfilms. Hence, materials with unique phases and properties can be createdin this manner.

[0070] 2. Nanowire Heterostructures.

[0071] Referring now to FIG. 2 and FIG. 3, an aspect of the presentinvention comprises the following two nanowire heterostructures asbuilding blocks for other heterostructures and devices: (i) a coaxialheterostructure nanowire (COHN) 10; and (ii) a longitudinalheterostructure nanowire (LOHN) 12. In the example shown in FIG. 2, COHN10 comprises a substantially crystalline core 14 surrounded by a sheath16 of a compositionally different material where a junction 18 is formedtherebetween. Sheath 16 can be substantially crystalline or amorphous, apolymer, semiconductor, oxide or the like. In the example shown in FIG.3, LOHN 12 comprises at least one segment 20 of a substantiallycrystalline material adjacent to at least one other segment 22 of acompositionally different material where a junction 24 is formedtherebetween.

[0072] Heterostructures according to the present invention can be formedwith any number of segments, both longitudinally and coaxially, and invarious configurations, some of which are described below.

[0073] For example, FIG. 3 shows a superlattice of additional segments26, 28, and 30, thereby illustrating that a heterostructure is notlimited to only two adjacent segments. It will be appreciated, however,that at least two of the segments should comprise compositionallydifferent materials in order to be a heterostructure. By“compositionally different” we mean (i) the materials have differentchemical compositions (whether intrinsic or doped) or (ii) the materialshave different crystal directions (e.g., the same materials butdifferent crystal orientation). The nanowire heterostructure couldcomprise compositionally different materials when viewed longitudinally,such as would be the case with alternating or periodic segments ofdifferent materials or multi-segmented nanowires where at least two ofthe segments comprise different materials. An example of a LOHN whereadjacent segments have different compositions would be a segment of Siadjacent to a segment SiGe.

[0074]FIG. 4 through FIG. 7 illustrate various examples of COHNs havingadditional segments. For example, FIG. 4 shows a COHN 32 having a core34 and a sheath comprising first and second segments 36, 38,respectively. FIG. 5 shows a COHN 40 having a core comprising first andsecond segments 42, 44, respectively, surrounded by a sheath 46. FIG. 6shows a COHN 48 having a core comprising first and second core segments50, 52, respectively, and a sheath comprising first and second sheathsegments 54, 56, respectively. FIG. 7 illustrates a COHN 58 having acore comprising a superlattice of segments 60, 62, 64, 66, 68, 70surrounded by a sheath 72. Note that the sheath can be crystalline oramorphous, and can include materials such as polymers, semiconductors,oxides, and the like. In addition, COHNs can have multiple sheathlayers.

[0075] In certain embodiments, COHNs can be formed by partially coatinga single-segment nanowire or a LOHN. For example, FIG. 8 shows a COHN 74having a single-segment core 76 which is only partially surrounded by asheath 78. FIG. 9 shows a COHN 80 having a LOHN core comprising segments82, 84 wherein the core is only partially surrounded by sheath 86.Alternatively, the cores could comprise superlattices with a partialsheath. Note also that the sheath portion could be segmented as welland, further, that the segments of the sheath could be adjacent orspaced-apart. Those skilled in the art will appreciate that these sheathconfigurations are achieved using conventional masking techniques, andthat these configurations represent just a few of the possibleconfigurations based on the nanowire structures described herein.

[0076] From the foregoing, it will be appreciated that segments ofheterostructures can comprise various materials, including, for example,semiconductor materials which are doped or undoped (i.e. pure intrinsicsemiconductors) and arranged to form a variety of semiconductor deviceswith junctions such as pn, pnp, npn, pin, pip and so forth. In certainembodiments the materials can be doped in a conventional manner. Forexample, conventional dopants such as B, Ph, As, In and Al can be used.Both the nanowire and the dopant materials can be selected from GroupsII, III, IV, V, VI, etc. and can include quaternaries and tertiaries, aswell as oxides.

[0077] One of the inventive aspects of the present invention is thatwhile it is commonly believed that such nanostructures cannot be“homogeneously doped” (i.e. doped such that dopant molecules aredispersed in a microscopically homogeneous manner), the materials of thepresent invention operate as if homogeneous doping was performed,because they conduct as would be expected if dopant molecules had beenhomogeneously distributed throughout the material. This result isunexpected since the high temperature and small size of the nanowireswould suggest that all dopant molecules would be annealed to the surfaceof the wires, where they would behave in the manner of trap sites ratherthan replicating the electronic properties of a “homogeneously doped”semiconductor.

[0078] In a number of embodiments, the present invention contemplatesnanowire heterostructures comprising one or more doped semiconductorsselected from a group that includes, but is not limited to, type II-VIsemiconductor, type III-V semiconductor, type II-IV semiconductor, andthe like.

[0079] Essentially any semiconductor material and its alloys can be usedas adjacent materials in a nanowire heterostructure according to thepresent invention. For example, FIG. 10 schematically illustrates ananowire heterostructure 88 which is a pn junction device 88. FIG. 11schematically illustrates a nanowire heterostructure 90 which is a pnp,npn, pin, pip, etc. junction device. Many of the nanowireheterostructures according to the present invention are generally basedon a semiconducting wire wherein the doping and composition arecontrolled in either the longitudinal or radial directions, or in bothdirections, to yield a wire that comprises compositionally differentmaterials.

[0080] As indicated above, in a heterostructure according to the presentinvention at least one of the segments comprises a material that issubstantially crystalline, particularly at its core. It will beappreciated that oxides on a nanowire surface can be amorphous withoutdestroying the substantially crystalline ordering of the nanowire core.In addition, the nanocrystals can include defects, substitutions ofatoms, certain dislocations, and combinations thereof, without defeatingsubstantial long-range ordering. In general, insofar as the materialexhibits substantial long-range ordering (e.g. ordering over a distanceof approximately 100 nm, it will be regarded as substantiallycrystalline and/or substantially monocrystalline. Insofar as thematerial exhibits long-range ordering, then the material is consideredto be substantially crystalline according to the present invention.Preferably, at least the inner 20% of the material from thecross-sectional center outward is substantially monocrystalline. In thecase of silicon nanowires, epitaxial growth is preferred (i.e.,monocrystalline growth for silicon on a silicon wafer by precipitatingsilicon from a vapor).

[0081] The diameter of a nanowire according to the present invention istypically less than approximately 200 nm at the maximum point ofdiameter and preferably in the range from approximately 5 nm toapproximately 50 nm. In addition, the variation in diameter across anensemble of wires synthesized in the same process is relatively sharp,such that the distribution of diameters is typically less thanapproximately 50%, preferably less than approximately 20%, morepreferably less than approximately 10%. In cases where the cross-sectionof the nanowire is not circular, the term “diameter” in this contextrefers to the average of the lengths of the major and minor axis of thecross-section of the nanowire, with the plane being normal to thelongitudinal axis of the nanowire.

[0082] In certain embodiments, nanowires according to the presentinvention typically exhibit a high uniformity in diameter from end toend. More particularly, over a section of the nanowire that shows themaximum change in diameter would preferably not exceed approximately10%, more preferably it would not exceed approximately 5%, and mostpreferably it would not exceed approximately 5%. The change in diametermay be considered to be given by (d_(max)−d_(min))/d_(min)). It shouldbe recognized by one of ordinary skill in the art that the ends of thenanowire will contain a sharp change in diameter, possibly evenexhibiting an infinite slope, wherein the measure described above isconsidered to be at a location away from the ends of the nanowires. Themeasurement preferably being made at a location separated from an end byat least 5%, and more preferably at least 10%, of the total length ofthe wire. In certain embodiments, the change in diameter is evaluatedover a length of the nanowire that ranges from approximately 1%,preferably up to approximately 25%, more preferably up to approximately75%, and most preferably up to approximately 90% of the total length ofthe nanowire.

[0083] The junctions between the compositionally different substantiallycrystalline materials defining nanowire heterostructures according tothe present invention typically exhibit a high degree of sharpness. Forexample, in accordance with the present invention, the transition zonebetween these materials can be made as sharp as approximately one atomiclayer to the total lateral length of the nanowire (i.e. a continuouslyvarying alloy along the length of the wire). Typically, the transitionshould be relatively sharp, however, the transition may span between asingle atomic layer and approximately 50 nm, and more preferably betweena single atomic layer and approximately 20 nm.

[0084] For the purposes of evaluating the length of a transition(transition zone) in the case of a LOHN, the beginning of the transitionzone transitioning from a first material to a second material can bedefined as a point along the longitudinal axis moving from the firstmaterial to the second material wherein the deviation in materialcomposition (e.g. dopant concentration and/or base material composition)of the first material is less than approximately 20%, more preferablyless than approximately 10%, more preferably less than approximately 5%,and most preferably less than approximately 1%. The end of thetransition zone transitioning from the first material to the secondmaterial can be defined as the point along the longitudinal axis movingfrom the first material to the second material where the deviation inmaterial composition of the nanowire at that point as compared to thecomposition (e.g. dopant concentration and/or base materialconcentration) of the second material is less than approximately 20%,more preferably less than approximately 10%, more preferably less thanapproximately 5%, and most preferably less than approximately 1%. In thecase of a COHN, the beginning and end of the transition zone aremeasured as a function of composition radially from the center of thenanowire. In either case, the transition zone should represent a changefrom a substantially crystalline and preferably substantiallymonocrystalline material to a compositionally different material. Itshould be appreciated, however, that since heterostructures according tothe present invention can comprise multiple segments eitherlongitudinally, coaxially, or both. It should also be appreciated thatit is also possible to form heterostructures in which some junctionsexhibit a high degree of sharpness while others do not, as depends uponthe specific application and requirements. Furthermore, not only can thecomposition of the materials forming adjacent segments be sharp orgradual, but by controlling the doping of materials forming segments ofthe heterostructure, it is possible to have sharp or gradual dopanttransitions between segments.

[0085] Referring again to FIG. 2, note that the band structure ofmaterials 14 and 16 in COHNs can be so chosen that one can achievemodulation doping, whereby the dopant atoms would reside in the sheath16 and the carriers would be generally confined in the core 14. Thiswill provide very high electron mobility due to reduced dopant andinterface scattering that has been observed in uncoated nanowires. Thisis the one-dimensional (1-D) version of the two-dimensional (2-D)electron gas that is created by semiconductor 2-D heterostructures. Sucha 1-D electron gas can then utilized, for example, in high-performancethermoelectric and photonic devices where electron mobility plays animportant role.

[0086] 3. Nanowire Synthesis.

[0087] Nanostructures with reduced dimensionality such as nanowires areboth fundamentally interesting and technologically important. Yet,nanowire synthesis has remained an enormous challenge for materialscientists because of the difficulty with one-dimensional control.Carbon nanotubes can also be used as templates to prepare nanorods ofdifferent compositions. There are also efforts using membrane templatesto make metal or semiconducting nanorods. However, these nanorods aremostly polycrystalline, which partly limit their potential usefulness.In order to gain well-defined structure-property correlation for these1D systems, it was necessary to develop general and predictivemethodology for the synthesis of single crystalline nanowires withuniform size and aspect ratio.

[0088] 3.1 VLS Mechanism.

[0089] The nanowires and nanowire heterostructures of the presentinvention can be synthesized by a wide variety of methods. In preferredembodiments, however, the nanowires are synthesized utilizing a modifiedvapor liquid solid (VLS) procedure. This process is described in detailin the examples provided herein, which are provided by way of exampleand not of limitation, wherein a number of modifications of theexemplified process are contemplated and within the scope of the presentinvention.

[0090] In contrast to the above synthetic approaches, thevapor-liquid-solid (VLS) process is a very powerful method to chemicallysynthesize single-crystalline 1D nanomaterials. This process, which hasbeen previously used to produce micron sized whiskers and recentlynanowires with various compositions, involves dissolving the gasreactants in nanosized catalytic liquid followed by one-dimensionalgrowth of single-crystalline nanowhiskers. The catalyst can be easilychosen based on the analysis of the equilibrium phase diagrams.

EXAMPLE 1

[0091] Referring to the schematic diagram in FIG. 12, an example of thegrowth process of an Si nanowire on an Si (111) substrate 100 isillustrated. In this example, SiH₄ gas 102 is utilized as the Si vaporsource and Au nanoclusters 104 as catalysts. The chemical vapordeposition (CVD) is preferably carried out at approximately 600° C. toapproximately 800° C. At this temperature, the Au nanoclusters 104 forma liquid alloy with Si and spontaneously break up into nanometer sizeddroplets 106 of Au—Si alloy. Next, the Si species continuously depositinto Au—Si alloy droplets where growth of the Si nanowire 108 isinitiated upon supersaturation of the gold by the silicon. The processcontinues until nanowire 108 achieves the desired length. Nanowires havebeen successfully prepared from Si, Ge and ZnO utilizing this mechanismin a conventional chemical vapor transport/deposition (CVT/CVD) system.Transmission electron microscopy (TEM) and X-ray diffraction (XRD)studies indicate that the inorganic nanowires are single crystallinewith a preferred growth direction (e.g. [111] for Ge). The diameter ofthese nanowires can be precisely controlled at diameters less thanapproximately 200 nm. Preferably the diameters are controlled atdiameters less than approximately 100 nm, more preferably less thanapproximately 50 nm, and most preferably less than approximately 25 nm,15 nm, or 10 nm. By using catalyst nanoclusters (e.g. Au, Co, Ni, Fe)with a monodisperse diameter distribution and with different sizes (e.g.from approximately 1 nm to approximately 100 nm, more typically fromapproximately 5 nm to approximately 100 nm), such nanowires can readilybe produced (e.g. having a diameter ranging from approximately 5 nm toapproximately 200 nm, and most typically from approximately 10 nm toapproximately 50 nm). These catalysts can be either dispersed on a Sisubstrate (e.g. an Si substrate or a substrate comprising other desiredmaterial or materials) or on top of a mesoporous silica film (e.g. an Sifilm or alternative films comprising other desired material ormaterials). It was also found that the aspect ratio of the nanowires canbe varied from approximately 1.5 or 2 to on the order of 1,000,000 andmore typically from appromximately 100 to approximately 100,000 by usingdifferent growth times.

[0092] The position of the nanowires on substrate 100 can be controlledthrough any convenient method of patterning the catalyst. Such methodsinclude, but are not limited to various sputtering and controlleddeposition techniques, various lithographic masking and/or etchingtechniques, along with additional methods and combinations thereof. Incertain embodiments, nanowire arrays can be fabricated bylithographically patterning a thin film catalyst on the substrate andheating the film until it melts into a plurality of droplets where eachdroplet acts as the catalyst for an individual nanowire.

[0093] Furthermore, the substrate material is not limited to Si, or evena single material, and for example, insulators such as sapphire can beused as a substrate. Generally any material that can be made soluble orsuspended in an appropriate catalyst may be utilized in the formation ofa nanowire by the methods of the present invention. Such materialsinclude, but are not limited to Group II, III, IV, V, and VI materialsor alloys thereof.

[0094] The metal catalyst can be a material other than Au as well, andalso need not be limited to a single material (e.g. the use of variousalloy materials is contemplated). By way of example, GaN nanowires canbe fabricated on a C-sapphire substrate using an Ni catalyst and a vaporof Ga and NH₃. Here, preferred growth would be in the (002) direction.Furthermore, the nanowire can be doped with Mn by using a mixture ofMnO₂ and C. Similarly, a Ga(Co)N nanowire can be grown by using an Nicatalyst, C-sapphire substrate and Ga+NH₃→Co doping by Co₃O₄+C mixture.A GaN nanowire can also be grown by using a Ni catalyst, C-sapphiresubstrate and Ga₂O₃+C mixture. Alloyed Ga—N—Zn—O nanowires can be grownby using an Ni catalyst, C-sapphire substrate and Ga+NH₃→layer by ZnO+Cmixture.

EXAMPLE 2

[0095] High temperature TEM was used to observe the growth of a Genanowire in situ. Here, small numbers of Ge particles were dispersed onTEM grids together with Au nanoclusters and served as the Ge vaporsource when the sample stage was heated in the vacuum chamber. It wasobserved that the melting of Au clusters initiated after the Ge—Au alloyformation. This was followed by an increase of the liquid droplet sizeduring the Ge vapor condensation process. When the dropletsupersaturates with the Ge component, the Ge nanowire spits out (isejected) from the alloy droplet and starts to grow. The real-timeobservation of the nanowire growth directly mirrors the mechanism shownin FIG. 12.

[0096] Based on these observations, several aspects of nanowire growthcontrol are immediately apparent:

[0097] (1) Inorganic nanowires with different compositions (e.g. Si, Ge,GaAs, CdSe, GaN, AIN, Bi₂Te₃, ZnO, and others) can be synthesized byusing suitable metal catalysts, gas precursors and reactiontemperatures. The latter can be determined by examining the binary orternary phase diagram

[0098] (2) Conventional dopants such as B, Ph, As, In and Al can beused.

[0099] (3) Materials can be selected from types III-V, II-VI, II-IV,etc. and can include quaternaries and tertiaries, as well as oxides.Essentially any semiconductor material and its alloys can be used asadjacent materials in a nanowire heterostructure according to thepresent invention.

[0100] (4) To the first order approximation, the nanowire diameter isdetermined by the catalyst size. Smaller nanoclusters will yield thinnernanowires. This has also been successfully demonstrated in the GaP andSi nanowire system.

[0101] Synthesis methods of the present invention share some features ofsurfactant-mediated epitaxial growth in that the mediating material (inthe form of a molten metal nanoparticle or a monolayer, respectively)catalyzes the epitaxial growth by inhibiting the reconstruction of thesemiconductor growth surface. Since there is no stable reconstructionthat must be continually disassembled and reestablished, nanowire growthcan occur selectively and at lower temperature than conventionalepitaxial growth. The lower temperatures offer the opportunity to accessnew phases, to produce sharper interfaces, and to inhibit morphologicalevolution of the nanowire material during the growth process (e.g.,Rayleigh breakup).

[0102] 3.2 Altered Phase Equilibria.

[0103] The nanowire geometry provides the opportunity to synthesizephases not stable in bulk or thin-film form. As a result of the highsurface-to-volume ratio, surface energy contributes more strongly to thetotal free energy terms in the free energy, especially for crystallinephases with highly anisotropic surface energies. For example, theequilibrium phase boundary between zincblende (cubic) and wurtzite(hexagonal) polytypes of III-V and II-VI semiconductors will shift inpressure and temperature relative to the bulk equilibrium boundary. Forexample, comparing a <111> oriented zincblende nanowire to a <0001>oriented wurtzite nanowire of the same composition, a cylindricalwurtzite nanowire will more closely approximate the equilibrium (Wulff)shape, exposing low surface-energy prismatic facets. The wurtzite phasehas indeed been observed as the preferred phase of GaAs in previousresearch on nanowire synthesis by OMCVD. In addition, epitaxialrelationship between the substrates and nanowires could also be utilizedto trap metastable phases in nanowire forms. This strategy has beensuccessfully used in thin film growth.

[0104] 3.3 Heteroepitaxy in Nanowires.

[0105] Semiconductor heterostructures enable confinement of electronsand holes, guiding of light and selective doping, yet these interfacesmust be dislocation-free if they reside in active regions of the device.The range of materials that can be grown by coherent epitaxy on a givensubstrate, and to a required thickness, is greatly limited by thelattice misfit. For a given lattice misfit, the equilibrium criticalthickness for coherent epitaxy may be estimated with knowledge of theelastic properties of the film and the core energy and crystallographyof a misfit dislocation (e.g., in-plane edge component of the burgersvector). Although coherent heteroepitaxial films can be grown wellbeyond the equilibrium critical thickness, the films are metastable torelaxation by dislocation mechanisms. The nanowire morphology producedby the methods of this invention provides the opportunity to markedlyextend both the equilibrium and kinetic critical thicknesses (orequivalently, the lattice misfit that can be accommodated at a giventhickness) due to the change in boundary conditions.

[0106] There are two primary effects. The first is the relaxation of theelastic boundary conditions normal to the growth direction. In the idealthin film morphology, the strain energy stored in the coherent film perunit area increases linearly with film thickness. In a nanowireheterostructure, the “film” is constrained laterally only at theinterface. As the nanowire “film” thickens, it will relax laterally sothat the stored elastic strain energy saturates. In fact, some of thestrain energy will also be stored on the “substrate” side, as thismaterial may relax laterally as well. The result is that the equilibriumcritical thickness for a given lattice mismatch will be extendedrelative to the thin-film value. Unlike the film case, there will be afinite range of lattice misfits that are associated with infinitecritical thickness due to the saturation in stored elastic strain energywith thickness. Countering this first effect is the fact that the strainenergy penalty associated with the misfit dislocation strain field isreduced due to the reduced volume of the nanowire. However, the coreenergy term remains and thus the first effect is expected to dominate.

[0107] 3.4 Longitudinal Heterostructure Nanowires (LOHNs).

[0108] The success of semiconducting integrated circuits is largelydetermined by the capability of defect engineering through controlleddoping. Defect engineering is expected to have even more profound effecton nanowires because not only will it contribute to doping and therebylead to novel devices, it may also strongly influence electronscattering.

[0109] Using the methods described herein, compositional profiles suchas shown in FIG. 10, FIG. 11, and FIG. 13 are generated along the wireaxis through successive feed-in of different dopant gas. For example, toachieve a LOHN comprising a Si pn-junction as illustrated in FIG. 13,species such as B₂H₆ and PH₃ would be sequentially used during nanowiregrowth. The CVD process allows accurate growth control of thecompositional profile and makes it possible to fabricate junctions withsharp compositional interfaces. 1-D superlattices 130 of Si/Ge andvarious III-V, II-VI, II-IV and tertiary and quaternary materials canalso be fabricated using this approach as illustrated in FIG. 14.Therefore, by sequentially changing the gas used in the VLS process,LOHNs can readily be synthesized. The process will, in general, allowbandgap engineering in 1-D and thereby also allow fabricating a sequenceof multiple quantum dots. Quantum dots are currently grown either insolution or through island formation during thin film growth. Becausethe location of these quantum dots is not known apriori, it becomes verydifficult to make contacts with individual quantum dots. By preciselyintegrating quantum dots within a single nanowire, the problems ofmaking contacts are eliminated. One thus obtains what we refer to as a“system on an nanowire.” These novel 1D nanostructures offer greatopportunities to explore new physics and phenomena for low dimensionalsystems. They can be potentially used as active nanoelectronic,nano-optical nanothermoelectric or nanoelectromechanical devices. It isalso possible to synthesize nanowires of different crystal structures,such as zinc blende and wurzite CdSe and GaN nanowires. This can beachieved by using different substrates to trap certain metastable phasesthrough the epitaxial growth relationship between the substrates andnanowires.

[0110] 3.5 Co-Axial Heterostructure Nanowires (COHNs).

[0111] It is also possible to synthesize co-axial nanostructures such asshown in FIG. 2 using the as-made nanowires as physical templates. Forexample, conformal and uniform carbon coating on Ge nanowires can beobtained by decomposing organic molecules on the wire surface. Thisapproach can be readily extended to create COHNs with strong electronconfinement effect. For example, GaAs nanowires fabricated using VLScould be subsequently coated with a thin layer of Al_(1−x)Ga_(x)As byusing low temperature chemical vapor deposition process that effectivelyavoids crystal growth along the wire axis and promotes surfaceovergrowth of Al_(1−x)Ga_(x)As. Note, however, that the sheath can becrystalline or amorphous, and can include materials such as polymers,semiconductors, oxides, and the like. To form a COHN, a single-segmentnanowire or a LOHN would first be formed according to any of the methodsdescribed herein. The single-segment nanowire or the LOHN, which willbecome the core of the COHN is then used as a template for forming thesheath. For example, the sheath can be formed by polymerization ofmonomers on the surface of the single-segment nanowire or the COHN.Alternatively, any physical vapor deposition (PVD) or chemical vapordeposition (CVD) process can be used to coat the single-segment nanowireor the LOHN. Examples of core/sheath materials, respectively, include,but are not limited to, Si and ZnO, Ge and C, Si and SiO₂, SnO2 andTiO₂, GaN and ZnO, GaAlN and GaN. Note that there is essentially anunlimited number of core/sheath material configurations. Even oxides,such as ZnO, can be used for the core material. The following is a listof core/sheath configurations where, for example, both the core and thesheath are monocrystalline: TiO₂/SnO₂; M:TiO₂/SnO₂ (M=Mn, Fe, Co, Cr,etc.); PbTiO₃/SnO₂; BaTiO₃/SnO₂; LaMnO₃/SnO₂; and HTSC/SnO₂ (hightemperature semiconductor—HTSC); GaAs/GaAlAs.

[0112] Note also that this approach can be used to synthesize ananotube. For example, a Ge nanowire core could be coated with anorganic molecular material. The surface of the organic material wouldthen be carbonized by pyrolysis in a vacuum. The Ge nanowire core wouldthen be melted or evaporated at a temperature ranging from approximately800° C. to approximately 1000° C., thereby forming a carbon nanotube. Inaddition, the same process may be utilized to form a “nanocylinder”, inwhich a COHN structure is formed and the core is then differentiallyetched away, leaving only the outer sheath (or cylinder). This cylindermay be made from any of the materials from which a sheath may be made,including but not limited to C, Si and SiO₂, SnO2 and TiO₂, GaN and ZnO,GaAlN and GaN. It will be appreciated that structural characterizationof these nanowires will rely heavily on transmission electron microscopy(TEM) and X-ray diffraction (XRD). Both XRD and TEM will allow fordetermining the structure/phase of the nanowires. In addition, TEM willprovide further information on the defect structures within individualwires, the local microstructure at the interface, growth direction, andoverall crystallinity.

[0113] 4. Nanowire Properties.

[0114] 4.1 Electronic Structure and Properties.

[0115] 4.1.1 Modeling.

[0116] The role of interface roughness and localization in nanowires hasbeen extensively studied in quantum wires defined using electron beamlithography or using electrostatic confinement with split gate method.The transition from ballistic to diffusive transport, positive andnegative magneto resistance, conductance quantization and universalfluctuations have been observed at low temperatures. Nanowiresfabricated using the process of the present invention provide a uniqueopportunity to study electron transport in a variety of 1D electronicmaterials. In addition, the possibility to dope the nanowires similar toCVD deposited thin film materials gives an extra degree of freedom toinvestigate dominant scattering mechanisms at various electrondensities. Referring to FIG. 2 and FIG. 15, in COHNs, modulation dopingof wider bandgap material will allow spatial separation of ionizeddopants 140 and free carriers 142, and thus higher mobilities could beachieved. Confining free carriers to the core region inside the nanowiresheath will reduce the surface scattering effects. When electrons occupycylindrical regions next to the heterostructure interface in coaxialnanowires, new quantized whispering gallery electronic states can alsobe formed. Referring to FIG. 16, heterostructures along the direction ofa LOHN nanowire will allow formation of quantum dot states 144. Thesestates can significantly affect electronic properties of nanowires. Onecould observe coulomb blockade as well as 1D resonant tunnelingfeatures.

[0117] In certain embodiments, modeling is preferably carried out in twostages. First, simple 1D band structure models and relaxation timeapproximation are used to estimate electron mobility along the nanowiresat higher temperatures. More elaborated models are then utilized, suchas variable range hopping, to take into account the surface/interfacescattering and calculate the temperature dependence of electricalconductivity. Other factors, such as modifications in phonon spectra andscattering times, electron-phonon interaction, and so forth may bestudied using Monte Carlo simulations of the Boltzmann equation. Notethat heterostructure nanowires can contain several confined andinterface phonon modes that can scatter electrons differently from whatone may find in bulk semiconductors.

[0118] 4.1.2 Characterization.

[0119] In order to characterize the electronic properties of bulk andheterostructure nanowires, it is important to measure the dopingconcentration profile along the nanowire, electron mobility, potentialbarrier at the hetero interfaces, etc. Conventional bulk or thin filmcharacterization methods have to be carefully examined before applyingto nanowire materials. Electrical conductivity along the nanowire is animportant parameter and should be characterized over a wide range oftemperatures. In addition, measurement of the magneto resistance willgive more information how the surface scattering affects electrontransport. Measurement of the thermoelectric properties (Seebeckcoefficient) will give more information about the features of theelectronic density-of-states and scattering mechanisms near the Fermisurface. Measurement of thermionic emission current can be used todetermine the heterostructure barriers along the nanowire direction.

[0120] Ballistic Electron Emission Microscopy (BEEM) is one of the idealtechniques to measure the “localized” electronic properties of nanowirestructures and characterize the coaxial heterostructures. BEEM is apowerful low energy electron microscopy technique for lateral imagingand spectroscopy (with nm resolution for buried structures placed up to30 nm below the surface). The BEEM technique has been used to study avariety of self-assembled quantum dot structures grown on GaAs.

EXAMPLE 3

[0121] GaSb quantum dots grown on GaAs were observed through STM andBEEM images. In the STM image, a roughly circular feature ˜50 nm indiameter and ˜5 nm tall, marked the lateral position of the buried dot.The area in the BEEM image aligned with the dot profile in STM wasdarker than the surrounding region, implying that the BEEM currentthrough the dot is reduced due to electrons reflection off the potentialbarrier of the dot. The height of this barrier (i.e. the local bandoffset) can be extracted from the changes in BEEM spectra between the onand off cases. The on dot and off dot BEEM spectra of several dots werefitted by using a modified Bell-Kaiser planar tunneling model, giving alocal conduction band offset for GaSb dots on GaAs of 0.08±0.02 eV. FIG.17 shows the band profile and FIG. 18 shows the characteristic BEEMspectra for GaSb/GaAs self-assembled single quantum dots.

[0122] In addition to measuring properties such as the heterojunctionband offset, the technique has been used to study the electronic bandstructure of new materials such as Ga As_(1−x) Nx alloys, the effect ofordering on the band structure of GaInP and resonant tunneling throughInP quantum dots confined between AlInP barriers.

[0123] It is clear that BEEM can be used to characterize the electronicproperties not only of the individual nanowires, but also the variationof longitudinal heterostructures of the type described herein, asillustrated in the configuration 150 of FIG. 19. Confinement effectswould lead to structure in the BEEM current which can be analyzedthrough second-derivative (SD) BEEM spectroscopy.

[0124] 4.2 Optical Properties.

[0125] Observing light emission from nanowires is extremely challengingbecause of the role of surface states and non-radiative recombination atthese states. With the use of coaxial heterostructure nanowires (COHNs)electrons are confined in the center regions inside the wire. The effectof free surfaces is thus reduced. Photoluminescence spectroscopy in awide range of temperatures can be used to study light emission fromnanowires, taking advantage of superresolution techniques in order toobtain images with subwavelength spatial resolution. Additionally,scanning solid immersion lenses can be used to characterize localizedlight emission from individual nanowires. Fabrication andcharacterization of pn junctions in nanowires is one of the key buildingblocks for optoelectronic devices. DC and pulsed electrical and opticalmethods can be used to measure photocurrent, recombination lifetime andelectroluminescence in nanowires.

[0126] 4.3 Thermal Properties.

[0127] Thermal properties of semiconductors are generally dominated bytransport of acoustic phonons. The thermal conductivity due to phononscan be related to two fundamental characteristics: (i) phonon dispersionrelation; and (ii) phonon lifetime. Thermal conductivity can becalculated using the relation$k = {\frac{1}{3}{\sum\limits_{p}\quad {\int{{v^{2}\left( {p,\varepsilon} \right)}{\tau \left( {r,p,\varepsilon} \right)}\frac{{df}_{BE}\left( {\varepsilon,T} \right)}{dT}\varepsilon \quad {D\left( {p,\varepsilon} \right)}{\varepsilon}}}}}$

[0128] where p is the phonon polarization, v(p,∈) is the group velocitywhich is a function of the polarization and energy, ∈=hω is the phononenergy, f_(BE)(∈,T) is the Bose-Einstein equilibrium distribution, T isthe temperature, D(p,∈) is the density of states, and τ(r,p,∈) is thephonon lifetime as a function of position, polarization and energy. Atroom temperatures (T=0.1θ_(D),θ_(D):Debye temperature), the thermalconductivities of most bulk semiconductors are limited by phonon Umklappscattering.

[0129] Phonon transport in nanowire heterostructures can be vastlydifferent from that in bulk semiconductors mainly because the dispersionrelation is significantly modified due to impose confinement in twodirections. Second, the presence of heterostructure interfaces introducephonon modes which exist at the interfaces. These result in manydifferent phonon polarizations other than the two transverse and onelongitudinal acoustic branches found in bulk semiconductors. Thesechanges in the dispersion relation modifies the group velocity and thedensity of states of each branch. The changes in phonon temperaturescome from two sources. First, the phonon-phonon interactions can changebecause selection rules based on energy conservation and wave-vectorrelations depend on the dispersion relation. Second, boundary scatteringcan be much stronger in nanowires (5-50 nm diameter) than in bulksemiconductors. Finally, because nanowire confinement can allow us toaccess new crystalline phases, the phonon dispersion relation can bedrastically modified.

[0130] The thermal and thermoelectric properties of nanowires accordingto the present invention can be measured using a microfabricatedstructure comprising two suspended heaters that contain e-beamlithographically fabricated wires. As a test, a multiwall carbonnanotube bundle was placed across the two heater sections so that thetwo heater sections were bridged. By monitoring the heat input from oneheater and the temperature of both heaters, the nanotube thermalconductivity was extracted. FIG. 20 plots the thermal conductivity as afunction of temperature of the multiwall carbon nanotube from 10° K to350° K, indicating a T² behavior suggestive of phonon confinement in a2-D material. The monotonic increase in thermal conductivity indicatessuppression of phonon-phonon scattering and the presence of very long(e.g., ≈1 μm) mean free paths. This approach can also be used formeasuring thermal conductivities of COHNs and LOHNs according to thepresent invention. In addition, batch-fabricated atomic force microscope(AFM) probes were employed with temperature sensors on the tip forscanning thermal microscopy (SThM) to thermally and thermoelectricallycharacterize COHNs and LOHNs locally.

[0131] In certain embodiments, nanowire characterization computationsfocus on three aspects: (i) calculation of phonon dispersion relations,(ii) calculations of phonon lifetimes based on dopant scattering,nanowire size and boundary scattering, and three-phonon enharmonicinteractions, and (iii) phonon transport calculations. Because waveeffects (phonon bandgaps) are already accounted for in the dispersionrelations, phase randomizing scattering may be assumed. Under thesecircumstances, the Boltzmann transport equation may be solved usingMonte Carlo simulations where it is simple to account for the density ofstates of different polarization branches in a nanowire, as well asfrequency dependent group velocities and phonon lifetimes.

[0132] 4.4 Thermoelectricity.

[0133] The thermopower of a semiconductor depends fundamentally on threeproperties: (i) the density of electronic states near the Fermi level,(ii) the electron effective mass, and (iii) carrier scattering rates.Because the electronic band structure (density of states and scatteringrate) can be dramatically changed by quantum confining the electrons ina nanowire, one could engineer the band structure and the position ofthe Fermi level in order to tailor the thermopower. The suspended heaterdevice described above can measure both temperature and potentialdifference across a nanowire. For example, FIG. 21 shows the thermopowermeasurements of multiwall carbon nanotubes in the 10° K to 350° Ktemperature range. The appearance of a positive thermopower indicatesholes as the dominant carriers in these carbon nanotubes. Therefore,this device can be used to measure thermopower of nanowireheterostructures such as COHNs and LOHNs described above.

[0134] 4.5 Piezoelectric Properties.

[0135] The wurtzite structure supports a spontaneous electric dipolemoment, and thus materials with this structure are both pyroelectric andpiezoelectric. These properties permit strong linear coupling betweenapplied mechanical stress and polarization (direct piezoelectriceffect), between applied electric field and strain (conversepiezoelectric effect), and between a change in temperature and a changein polarization (pyroelectric effect). Wurtzite nanowires (e.g., GaAs,InAs, GaN, AIN, ZnO, etc) and nanowire heterostructures are thuspotentially useful as sensors and actuators at the nanoscale. Potentialapplications include integrated atomic force microscopy probes, resonantmass sensors with single-molecule sensitivity, nanoscale thermalsensors, electric-field-tunable GHz filters, large displacement nanobeamactuators, and nanoscale flow sensors.

[0136] In <0001> wurtzite nanowires, the spontaneous polarization isoriented along the wire axis. Thus, electric fields and metal mechanicalstress applied along the wire axis will generate the largestpiezoelectric response. The simplest electrode configuration utilizescontacts at the base and at the tip. Longitudinal stress applied withthe tip and base contacts will be sensed by the direct piezoelectriceffect. Since the wire cross-sectional area is small, large stresses canbe generated with small forces. With the nanowire used as a resonantsensor, one end of the nanowire must be mechanically free, and aconductive surface in close proximity will be used to detect charge onthe nanowire tip and to remove or add charge by tunneling.

[0137]FIG. 22 shows an experimental setup to measure the mechanicalmotion of a piezoelectric or pyroelectric nanowire 160 on a conductingsubstrate 162 using an AFM cantilever probe 164 while simultaneouslymeasuring the electrostatic potential across the nanowire with a voltagesensor 166. The tip of the AFM probe contacts the metal catalyst “cap”168 on the nanowire for electrical and mechanical measurements.

[0138] 5. Block-by-block Growth of Single-crystalline Si/SiGe.

[0139] Heterojunction and superlattice formation is essential for manypotential applications of semiconductor nanowires in nanoscaleoptoelectronics. Accordingly, we have developed a hybrid Pulsed LaserAblation/Chemical Vapor Deposition (PLA-CVD) process for the synthesisof semiconductor nanowires with longitudinal ordered heterostructures.The laser ablation process generates a programmable pulsed vapor source,which enables the nanowire growth in a block-by-block fashion withwell-defined compositional profile along the wire axis. Singlecrystalline nanowires with longitudinal Si/SiGe superlattice structurehave been successfully synthesized. This unique class ofheterostructured one-dimensional nanostructures holds great potential inapplications such as light emitting devices and thermoelectrics.

[0140] The success of semiconductor integrated circuits has been largelydependent upon the capability of heterostructure formation throughcarefully controlled doping and interfacing. In fact, 2-dimensional (2D)semiconductor interface is ubiquitous in optoelectronic devices such aslight emitting diode, laser diodes, quantum cascade laser andtransistors. Heterostructure formation in 1-dimensional (1D)nanostructures (nanowires) is equally important for their potentialapplications as efficient light emitting sources and betterthermoelectrics. While there are a number of well-developed techniques(e. g. molecular beam epitaxy) for the fabrication of thin filmheterostructures and superlattices, a general synthetic scheme forheterojunction and superlattice formation in 1D nanostructures withwell-defined coherent interfaces is currently still lacking. Here, ahybrid Pulsed Laser Ablation/Chemical Vapor Deposition (PLA-CVD) processis described for the synthesis of semiconductor nanowires with periodiclongitudinal heterostructures. Monocrystalline nanowires with Si/SiGesuperlattice structure were obtained and thoroughly characterized usingelectron microscopy.

EXAMPLE 4

[0141] Referring FIG. 23, an embodiment of a nanowire growth apparatus90 in accordance with the present invention is illustrated. Growthapparatus 170 comprises a furnace 172 having a quartz reaction tube 174.A (111) Si wafer 176 coated with a thin layer of Au was put inside thequartz reaction tube 174 as substrate. A gas mixture of SiCl₄ and H₂ wascontinuously introduced into the reaction tube 174 through an inlet 178.A computer programmed laser pulse 180 was focused on a pure Ge target182. Residue gas was exhausted through an outlet 184.

[0142] Referring now to FIG. 23 and FIG. 24 together, nanowire growthusing Au as a metal solvent at high temperature as previously describedand illustrated in FIG. 12. This process starts with the dissolution ofgaseous reactants in nanosized liquid droplets of the metal solvent,followed by nucleation and growth of single crystalline wires. Theconcept of heterostructured nanowires requires accurate compositionalprofile and interface control at nanometer or even atomic level whilemaintaining a highly crystalline and coherent interface along the wireaxis. Based on our fundamental mechanistic understanding of VLS nanowiregrowth, this level of control is made possible here through successivefeed-in of different vapor sources.

[0143] In one embodiment of the present invention, the size andsize-distribution of the nanowire heterostructures can be controlled byusing preformed and size controlled nanocrystal catalysts to form thenanowires.

[0144] Referring to the process flow diagram of FIG. 24, in the exampleillustrated Si/SiGe superlattice nanowires were synthesized bygenerating a Ge vapor in pulsed form through the pulsed ablation of thepure Ge target 182 with a frequency-doubled Nd-YAG laser (wavelength 532nm, 6 Hz, power density 10 J/cm² per pulse). The flow rate of the H₂ wasapproximately 100 sccm, the ratio of SiCl₄ and H₂ was approximately0.02, and the system pressure was atmospheric pressure. The reactiontemperature typically ranged from approximately 850° C. to approximately950° C. At this temperature, the Au thin film 186 forms a liquid alloywith Si and spontaneously breaks up into nanometer sized droplets ofAu—Si alloy 188. Next, the Si species continuously deposit into Au—Sialloy droplets where growth of the Si nanowire 190 is initiated uponsupersaturation. When the laser is turned off, only Si species depositinto the alloy droplet and a pure Si block is grown. However, if thelaser is turned on during the growth process, Ge vapor will be generatedand both Ge and Si species will be deposited into the alloy droplets.When the laser is turned on, SiGe alloy 192 then precipitates out fromthe solid/liquid interface. By periodically turning the laser on and off(this sequence can be easily programmed), an Si/SiGe superlattice 194 isformed on every individual nanowire in a block-by-block fashion. Theentire growth process resembles the living polymerization synthesis ofblock copolymer.

[0145] It will be appreciated that various other nanowire structures canbe grown using different gases and targets. For example, PbSe can begrown by laser ablation of a PbSe/Au target in Ar gas. Furthermore,growth of a nanowire superlattice according to the present invention isnot limited to the foregoing synthetic process. One alternative approachwould be to use multiple target materials and steer the laser with acomputer for selection of target materials. Additionally, essentiallyany physical or chemical vapor deposition process that uses vaporsupplies could be used, including, but not limited to, PLD, CVD, andMBE. For example, the vapor supplies could be configured with computercontrolled valves to pulse the flow of the desired gas.

EXAMPLE 5

[0146] A nanowire array 200 as depicted schematically in FIG. 25 wassynthesized using the process described in Example 4 and a scanningelectron microscopy (SEM) image of the synthesized nanowire array wasobtained. In the example shown, an Au film having a thickness on the Si(111) substrate 202 of 20 nm was lithographically pattered into foursites. Each film site melted into four droplets which acted as thecatalyst for a corresponding nanowire. During the growth process, thelaser was periodically turned on for 5 seconds and off for 25 seconds,and the cycle was repeated for 15 min. As previously shown, Si nanowiresgrow preferably along [111] direction, which results in the orientedepitaxial nanowire array growth on the Si (111) substrate. The alloydroplets solidify and appear as a bright spot on the tip 204 of everynanowire 206. Close examination of the nanowires showed the tips ashaving a flower-like shape which is formed during the solidification ofthe liquid alloy droplet. The diameters of these nanowires ranged fromapproximately 50 nm to approximately 300 nm. Using a Philip CM200transmission electron microscope (TEM) operated at 200 KeV, scanningtransmission electron microscopy (STEM) images of two nanowires inbright-field mode showed dark stripes appearing periodically along thewire axes, which originated from the periodic deposition of SiGe alloyand Si segments. The electron scattering cross section of Ge atom islarger than that of Si. Consequently, the SiGe alloy block appearsdarker than the pure Si block. The chemical composition of the darkerarea was examined using energy-dispersive X-ray spectroscopy (EDS) whichshowed a strong Si peak and apparent Ge doping (˜12 weight % Ge) asillustrated in FIG. 26. The periodic modulation of Ge doping was furtherconfirmed by scanning a focused electron beam along the nanowire growthaxis and tracking the change of X-ray signal from Si and Ge atoms in thewires as illustrated in FIG. 27. Both Si and Ge X-ray signals showedperiodic modulation and intensities that were anti-correlated; in otherwords, wherever the X-ray signal from Ge showed a maximum, the signalfrom Si showed a minimum, which confirms the formation of Si/SiGesuperlattice along the wire axis. We noted that the abruptness of theSi/SiGe interface in these nanowires is not ideal at this stage. It isbelieved that this could be improved by incorporating more precise andfaster vapor dosing/switching schemes such as molecular beam process.

[0147] It must be emphasized that the elastic boundary conditions ofheteroepitaxial growth offer the possibility to create dislocation-freeinterfaces in the superlattice nanowires that are not stable in theconventional 2D configuration achieved by epitaxial film growth onplanar substrates. Although coherent heteroepitaxial films can be grownwell beyond the equilibrium critical thickness, the films are metastableto relaxation by dislocation mechanisms. The VLS nanowire morphologyprovides an opportunity to markedly extend both the equilibrium andkinetic critical thicknesses—or equivalently, the lattice misfit thatcan be accommodated at a given thickness—due to the change in boundaryconditions.

[0148] The highly crystalline nature of our superlattice nanowires wascharacterized by selected area electron diffraction (SAED) andhigh-resolution transmission electron microscopy (HRTEM). An SAEDpattern was recorded perpendicular to a nanowire long axis. The patternwas then indexed as the diffraction along the [110] zone axis ofcrystalline Si and suggested the nanowire growth does occur along [111]direction. This was further confirmed in the HRTEM image which clearlyshowed the (111) atomic planes (separation 0.314 nm) perpendicular tothe nanowire axis. While the interface contrast was readily seen in theSTEM images, we were not able to resolve the interface in HRTEM mode dueto low doping percentage in SiGe blocks. These HRTEM images, however,clearly demonstrated the high crystallinity of the Si/SiGe superlatticenanowires. Extensive HRTEM imaging indicated that the mono-crystallinityof the Si/SiGe superlattice nanowire is maintained along the entire wirelength with few linear or planar defects.

[0149] Taken together, the structural and chemical composition datashowed that nanowires prepared by the PLA-CVD method according to thepresent invention are highly crystalline with Si/SiGe superlatticestructure along the nanowire axis. The diameters of the nanowires, theconcentration of Ge and the period of chemical modulation can be readilycontrolled by adjusting the reaction conditions. The nanowire diameteris influenced by the thickness of Au layer on substrate. For example,with 20 nm thick Au thin films, the average diameter of nanowires isaround 100 nm. If the thickness of Au is reduced to 1 nm, the averagediameter can be reduced to 20 nm. The diameter is also affected by thereaction temperature, wherein lower temperatures result in thinnernanowires. The concentration of Ge in the superlattice is controlled bythe ratio of Ge atoms and Si atoms deposited into the alloy droplets.Increasing the laser intensity or decreasing the flow rate of SiCl₄ canincrease the concentration of Ge. In addition, the superlattice period(L) is the product of growth rate (V) and laser on-and-off period (T):L=V×T. Therefore, by reducing the growth rate or the laser on-and-offperiod, we are able to reduce the superlattice period. Similarly, theratio of different compositional blocks can be readily adjusted byvarying the laser on/off ratio.

[0150] Importantly, by putting these “labels” along the wire growthaxis, the PLA-CVD process provides a quantitative way to measure thegrowth rate of nanowires (V=L/T) and its correlation with growthsupersaturation. While the laser on-and-off period T is preset, knowingthe superlattice period L, the growth rate V can be accuratelycalculated. We found that the growth rate is diameter-dependent underthe same reaction conditions. The smaller the nanowire diameter, theslower is the growth rate as illustrated in FIG. 28 which shows thecorrelation between growth rate and diameter observed in our experiment.The trend can be qualitatively explained by the Gibbs-Thomson effect,such as increasing Si vapor pressure and thereby decreasingsupersaturation as the nanowire diameter becomes smaller. The decreaseof supersaturation as a function of nanowire diameter (d) is given as$\frac{\Delta \quad \mu}{kT} = {\frac{{\Delta\mu}_{0}}{kT} - {\frac{4{\Omega\alpha}_{vs}}{kT}\frac{1}{d}}}$

[0151] where Δμ is the effective difference between the chemicalpotentials of Si in the nutrient (vapor or liquid) phase and in thenanowire, Δμ0 is the same difference at a plane interface, α_(vs) is thespecific free energy of the nanowire surface and Ω is the atomic volumeof Si. The dependence of the crystal growth rate V on thesupersaturation is generally non-linear and in many cases is of nthpower: $V = {b\left( \frac{\Delta\mu}{kT} \right)}^{n}$

[0152] where b is a coefficient independent of the supersaturation. Thisnaturally leads to a linear dependence between V^(1/n) and 1/d as in$\sqrt[n]{V} = {{\frac{{\Delta\mu}_{0}}{kT}\sqrt[n]{b}} - {\frac{4{\Omega\alpha}_{vs}}{kT}\sqrt[n]{b}\frac{1}{d}}}$where$\frac{{\Delta\mu}_{0}}{kT} = {\frac{4{\Omega\alpha}_{vs}}{kT}\frac{1}{d_{c}}}$

[0153] and d_(c) is the critical diameter.

[0154] Our Si/SiGe nanowire growth data can be readily fitted with n=2.This observation agrees well with the classical CVD crystal growthstudies on micrometer-sized Si whiskers by Givargizov.

[0155] The hybrid PLA-CVD methods described herein can be used toprepare various other heterostructures on individual nanowires in a“custom-made” fashion since part of the vapor source supplies (laserablation) can be programmed. It will enable the creation of variousfunctional devices (e.g. p-n junction, coupled quantum dot structure andheterostructured bipolar transistor) on single nanowires. Thesenanowires could be used as important building blocks for constructingnanoscale electronic circuit and light emitting devices. As an example,superlattice nanowires with reduced phonon transport and high electronmobility are believed to be better thermoelectrics.

[0156] 6. Nanowire-based Energy Conversion Devices.

[0157] Those skilled in the art will appreciate that the nanowiresdescribed herein may be used in a wide variety of applications,including, but not limited to, (a) thermoelectric refrigerators; (b)light emitting diodes; and (c) electromechanical sensors. The design ofthese devices flows directly from the fundamental scientificunderstanding of the effect of 1-D confinement on various physicalproperties. Although such scientific understanding can rely on singlenanowire studies, it will be appreciated that devices will requiremultiple nanowires for integration into systems. Therefore, arrays ofnanowires would typically be employed.

EXAMPLE 6

[0158] For purposes of discussion we will focus on the three devicesstated above; however, they are by no means the only devices possibleusing nanowires.

[0159] 6.1 Thermoelectric Refrigeration and Power Generation.

[0160] Solid-state refrigeration and power generation can be achievedusing the Peltier effect, whereby a current flow across thermoelectricjunctions produces can produce cooling (or heating). Conversely, atemperature difference across a thermoelectric material generates acurrent flow across a potential drop, and thereby electrical power.Compared to current vapor-compression refrigerators and gas-basedengines, such solid-state devices are extremely promising because: (i)they do not contain any moving parts; (ii) they are environmentallybenign; and (iii) allow for miniaturization. The reason they are notcurrently widely used is because their performance (efficiency forengines and coefficient of performance (COP) for refrigerators) is muchinferior to gas/vapor based systems. If, however, the performance couldbe improved to be comparable or better than the vapor based systems, onecould envision a drastic change in how we utilize or convert energy.This provides a strong and compelling reason to develop thermoelectricdevices based on nanowires. As discussed below, this can only beachieved using nanowires according to the present invention.

[0161] The materials used for solid-state thermoelectric refrigeratorsand power generators are characterized by a figure of merit, ZT=S²σT/k,where S is the thermopower, k is the thermal conductivity, σ is theelectrical conductivity, and T is the absolute temperature. Bi₂Te₃ andits alloys are currently the most widely used materials and have a ZT=1.It can be theoretically shown that if ZT=3, the performance ofthermoelectric refrigerators and engines can be comparable to those ofvapor compression ones. In fact, if thermoelectric materials arenanostructured, quantum confinement of electrons and phonons candrastically increase their ZT, as illustrated in FIG. 29. 1-D nanowiresin particular could reach ZT≈2 to 5 if the wire diameter lies in therange of 5 nm to 10 nm.

[0162] 6.1.1 Nanowire Design.

[0163] Because high electron mobility provides high ZT, it is preferablyto use COHNs since they will have much reduced dopant and interfacescattering. The thermopower of COHNs can be tailored through bandgapengineering. Because thermal conductivity of materials is generallyinversely proportional to the atomic mass (ζ), a high-ζ material wouldbe the material of choice. It is for this reason that Bi or Bi₂Te3nanowires are good candidates for thermoelectric applications. Thethermal conductivity can be further reduced by decreasing the nanowirediameter since boundary scattering is expected to be dominant fornanowire diameters less than 20 nm at room temperatures. In addition toBi2Te3, other materials can be used, such as SiGe or InGaAs where alloyscattering can reduce phonon transport.

[0164] 6.1.2 Device Design.

[0165] Because nanowires are fragile, they should be embedded in amatrix to provide mechanical strength. For example, arrays of Bi₂Te₃ orSiGe COHN nanowires would be embedded in a polymer or dielectricmaterial as illustrated in FIG. 30 to form a thermoelectric device 210.The thermoelectric device 210 in FIG. 30 comprises upper and lowerelectrically insulating substrates 212, 214, respectively, an n-dopednanowire array 216 that is separately grown on a substrate 218 andwherein the nanowires 220 are embedded in a polymer matrix 222, and ap-doped nanowire array 224 that is separately grown on a substrate 226and wherein the nanowires 228 are embedded in a polymer matrix 230. Thewafers of n- and p-doped thermoelectric nanowire arrays are the broughttogether and bonded to with a series electrical connection and aparallel thermal connection to make a thermoelectric cooler or powergenerators. These connections are achieved by forming and connectingmetal contact pads 232, 234, 236, 238 and 240 as shown. The nanowirearrays can be easily embedded in a polymer matrix by flowing a polymersolution after the nanowires are fabricated, and then curing it usingheat or UV radiation. To make the upper contacts 234, 240 (i.e., at thetips of the nanowires), the polymer will be preferentially etched downtill the nanowires are exposed, after which metal contact pads aredeposited.

[0166] The design parameters for such a composite are: (a) surfacedensity of nanowires; and (b) thickness of the device. The idea is toexploit the ultra-low thermal conductivity of polymers (k≈0.1 W/m-K) andthe high power factors (S²σ) of the nanowires in order to achieve highZT. Device performance can be characterized by measuring (a) effectiveelectrical conductivity of the device; (b) effective thermalconductivity of the device; (c) effective Seebeck coefficient; (d)temperature difference across the device in response to a current flow;and (e) electrical power in response to a temperature difference or heatflow rate.

[0167] 6.2 Light-Emitting Devices.

[0168] Nanowire composite materials have two distinct properties thatcan be used for light emitting device applications. On one hand, the lowdimensional confinement of electrons and quantization of energy levelscan be used for tuning of absorption and emission wavelength. The 1Dnature of crystal growth along the nanowire could permit higherflexibility in the lattice mismatch between different materials and thuswider change in absorption and emission spectra. On the other hand, Siand III-V semiconductors have an index of refraction (3-4) that is muchhigher than air or silica fiber (1-1.5). This creates a mode sizemismatch that is one of the main difficulties to couple light betweenfibers and semiconductor devices. This also limits the external quantumefficiency of light emitting diodes, since most of the emitted photonsare reabsorbed in the material.

[0169] Based on the scientific understanding of the electronic bandstructure of various II-V and II-VI nanowires, nanowires can be designedfor efficient absorption and emission of photons. For example, referringFIG. 31, a nanowire-polymer composite array 250 can be made byintegrating a plurality of semiconductor nanowires 252 into a polymermatrix 254 as previously described, thus producing an optically activematerial with a much lower effective index. The change in polymerrefractive index with temperature is typically an order of magnitudehigher than conventional semiconductors. This large thermo-opticcoefficient can be combined with electronic-electronic properties ofsemiconductor nanowires to produce novel energy conversion devices.

[0170] Preferably, nanowires with the highest radiative efficiencies areintegrated inside the polymer matrix in order to fabricate andcharacterize light emission devices. In addition, by using a mixture ofnanowires made from different materials, one can achieve wider emissionspectra and white light behavior.

[0171] Referring to FIG. 32, an electron ejection light emittingdiode/laser diode 260 is shown schematically as comprising a nanowire262 having pn junction 264 formed from growth of an n-type semiconductor266 and a p-type semiconductor 268. A positive electrode 270 andnegative electrode 272 are attached to the n- and p-type materials,respectively. Application of a potential across the electrodes willcause light emission by electron ejection as depicted in FIG. 32. Thisstructure can be formed using, for example, ZnO, Si/Ge, and GaN withappropriate dopants.

[0172] Referring to FIG. 33, using longitudinal heterostructurenanowires (LOHNs) 280, one can make single quantum dot LEDs and studythe effect of quantum dot size and material on the emission spectrum.The unique geometry of the quantum wire allows delivery of electrons andholes directly to the dots 282 and thus avoids the paths where electronsand holes would recombine in other places. Quantum dots can be formedusing, for example, Si/Ge, PbSe/PbTe, arid Bi₂Te₃/Sb₂Te₃. One could evenput the composite nanowire-polymer medium inside a vertical cavitydistributed dielectric mirror to provide for optical feedback and studythe stimulated emission and laser action.

[0173] Additionally, sophisticated T-shaped, V-grooved and Ridge quantumwire lasers and also quantum dot lasers have can be fabricated andcharacterized. These devices have unique properties due to 1D and 0Dnature of electronic density of states. In particular, the increase inthe differential gain will improve the high speed modulationcharacteristics. Size variation is also used to change electronic energystates and the emission spectrum. We expect that light emission in COHNsand LOHNs will create a new class of energy conversion devices whereopto-electronic properties can be tailored beyond what can be achievedwith current methods.

[0174] 6.3 Nanowire Device Flexibility.

[0175] As can be seen, therefore, nanowires according to the presentinvention can be used for fabricating a variety of devices in additionto those described above, including, but not limited to:

[0176] (a) High electron mobility nanowires (using COHNs).

[0177] (b) High electron mobility nanowire field-effect transistors(using COHNs and applying external bias to deplete/enhance a channel).

[0178] (c) Nanowire based infrared detectors (using LOHNs and embeddedquantum dots).

[0179] (d) Nanowire based 1D resonant tunneling diodes (using LOHNs andembedded quantum dots).

[0180] (e) Nanowire based single electron transistors (using LOHNs andembedded quantum dots and possible combination with COHNs).

[0181] (f) Nanowire based infrared detectors (using COHNs and quantizedwhispering gallery electron modes).

[0182] (g) Nanowire magnetic sensors (using COHNs and the quantizedwhispering gallery electron modes which are affected under a magneticfield).

[0183] (h) Polymer-nanowire composite light emitting devices (highexternal quantum efficiency, broad spectrum, good coupling with fiber).

[0184] (i) Polymer-nanowire composite optical modulators (can make veryhigh speed traveling wave modulators because the speed of electrical andoptical signals could be matched).

[0185] (j) Polymer-nanowire composite optical detectors.

[0186] (k) Polymer-nanowire composite waveguides and couplers (bygrowing nanowires with directional channels between the nanowires).

[0187] (l) Polymer-nanowire composite optical switches.

[0188] (m) Polymer-nanowire composite lasers (edge emitting, distributedfeedback or vertical cavity structures).

[0189] It will also be appreciated that LOHNs can be used to fabricatemulti-terminal devices (i.e., N>2) such as pnp devices. FIG. 34 shows anexample of a 3-terminal pnp LOHN 290 fabricated with a p-type material292, n-type material 294, and p-type material 296 and having terminalsT₁, T₂ and T₃. FIG. 35 illustrates another example of a 3-terminal LOHN300.

[0190] 6.4 Nanoelectromechanical Devices.

[0191] Nanowire pyroelectric and piezoelectric devices have, inprincipal, the following inherent features that distinguish them fromfilm or bulk devices:

[0192] (a) High quality factors: The lack of extended defects shouldenable high mechanical quality factors in nanowire resonators. The lowdefect density also suggests low loss tangents and thus a highelectromechanical coupling figure-of-merit (proportional to (tan δ)⁻¹).

[0193] (b) High surface-to-volume ratio: The low mass per unit length,combined with the high ratio of adsorption sites to the nanowire volumewill permit resonant detection of mass increments at sensitivitiesapproaching the single molecule level.

[0194] (c) Variable length without change in materials quality: Nanowirelongitudinal resonators can be made at various lengths from submicron totens or hundreds of microns, thus allowing the fabrication of sensors oractuators with a wide range of fundamental resonant frequencies.

[0195] (d) Nanoscale Diameter: The small diameter permits the use ofpiezoelectric and pyroelectric nanowires as direct probes of forces atthe atomic and molecular scale, and temperature at the nanoscale.Furthermore, “nanobeam” unimorph benders with longitudinal electrodesand elastic layers fabricated by shadow evaporation will be capable ofvery large deflections clue to length: thickness aspect ratiosapproaching 1000:1 and the large transverse electric fields possiblewith moderate voltages (e.g., 100 MV/m for 1 V applied across a 10 nmthick nanobeam).

[0196] Referring to FIG. 36 through FIG. 39, examples of nanowire deviceconfigurations for electromechanical transducers are illustrated. FIG.36 and FIG. 37 illustrate a longitudinal configuration, while FIG. 38and FIG. 39 illustrate a transverse configuration.

[0197] Referring first to FIG. 36 and FIG. 37, in <0001> wurtzite, thespontaneous polarization is longitudinal, i.e., oriented along the wireaxis. Thus, electric fields and mechanical stress applied along the wireaxis will generate the largest piezoelectric response. In thelongitudinal configuration 310, the simplest electrode configurationutilizes contacts (electrodes) 312, 314 at the base and at the tip,respectively. Longitudinal stress applied with the tip and base contactswill be sensed by the direct piezoelectric effect. Since the wirecross-sectional area is small, large stresses can be generated withsmall forces. For example, a uniaxial tensile force of 100 nNcorresponds to a uniaxial stress of 100 MPa for a wire with across-sectional area of (10 nm)². A piezoelectric coefficient of 5 nC/Nwill thus generate a change of polarization of 0.5 C/m², a value that iscertainly detectable. With the nanowire used as a resonant sensor, oneend of the nanowire must be mechanically free, and a conductive surfacein close proximity will be used to detect charge on the nanowire tip andto remove or add charge by tunneling.

[0198] Referring now to FIG. 38 and FIG. 39, a completely distinctsensing and actuation capability can be expected from wurtzite nanowiresin <hki0> orientation, that is, with the spontaneous polarizationperpendicular to the wire growth direction. Such nanowires can be grownby choosing the appropriate surface orientation of the single-crystalsubstrate, e.g. sapphire with substrate with (0001) or (hki0)orientation. In this transverse configuration 320, electrodes may beplaced at the wire ends, thus activating the piezoelectric shear mode,d₁₅, or along the wire length, employing d₃₁. In the d₃₁ mode shown inFIG. 38 where electrodes 322, 324 are placed along the wire length, itis possible to exploit the expected large electric breakdown strength(>300 MV/m) and high fracture strength of defect-free AIN nanowires tofabricate both high-displacement nanobeam unimorph benders and forcesensors. Presuming that a suitable elastic layer is shadow deposited onone side of the nanowire, opposite an electrode stripe, the tipdisplacement, δ, of the nanowire will be on the order of d₃₁L²V/t². Fortransverse voltage of 1V, thickness of 10 nm, length of 5 μm and d₃₁ of3 pm/V, the tip displacement is expected to be about 0.75 μm.

[0199] The transverse configuration represents synthesis and processingchallenges beyond those expected for the longitudinal configuration. Forexample, the nanowire must be nucleated in the transverse orientation,which will likely require nucleation on a crystalline wurtzite substrateor seed. Once nucleated, the surface energy anisotropy is expected toyield nanowires of rectangular cross section, ideal for shadowdeposition. Based on the substantial literature on lateral growthexperiments with GaN, it is likely that the growth rate of transversenanowires will greatly exceed that of longitudinal wurtzite nanowires.Once synthesized, nanobeam unimorph benders may be fabricated by shadowdeposition of metallic layers—one thin compliant metallization to serveas an electrode (e.g., Cr/Au), and a second stiffer layer on theopposite side (e.g., Ti/Pt) to serve both as an electrode, and as anelastic layer to optimize the position of the neutral axis forbending-mode actuation. Alternatively, it is possible to make use of thesubstantially different surface properties of the opposing basal facesto selectively deposit metals by solution processing.

[0200] 6.5 Room-Temperature Ultraviolet Nanowire Nanolasers.

[0201] Development of short-wavelength semiconductor lasers is of greatcurrent interest. This has culminated in the realization ofroom-temperature green-blue diode laser structures with ZnSe andln_(x)Ga_(1−x)N as the active layers. ZnO is another wide band-gap (3.37eV) compound semiconductor that is suitable for blue optoelectronicapplications. In fact, ultraviolet lasing action has been reported indisordered ZnO particles and thin films. For wide band-gap semiconductormaterials, a high carrier concentration is usually required in order toprovide an optical gain that is high enough for lasing action in anelectron-hole plasma (EHP) process. Such EHP mechanism, which is commonfor conventional laser diode operation, typically requires high lasingthresholds. As an alternative to EHP, excitonic recombination insemiconductors can facilitate low-threshold stimulated emission becauseof its bosonic nature. To achieve efficient excitonic laser action atroom temperature, exciton binding energy (E^(b)ex) has to be much largerthan the thermal energy at room temperature (26 meV). In this regard,ZnO is a good candidate since its E^(b)ex is approximately 60 meV, whichis significantly larger than that of ZnSe (22 meV) and GaN (25 meV).

[0202] To further lower the threshold, low-dimensional compoundsemiconductor nanostructures have been fabricated, in which quantum sizeeffects yield a substantial density of states at the band edges andenhance radiative recombination due to carrier confinement. The use ofsemiconductor quantum well structures as low-threshold optical gainmedia represents a significant advancement in semiconductor lasertechnology. Stimulated emission and optical gain have also beendemonstrated recently in Si and CdSe nanoclusters and their ensembles.

[0203] In accordance with a further aspect of the invention, we havedemonstrated the first excitonic lasing action in ZnO nanowires with athreshold of 40 kW/cm² under optical excitation. The chemicalflexibility as well as one-dimensionality of the nanowires makes themideal miniaturized laser light sources. These short-wavelengthnanolasers could have myriad applications including optical computing,information storage, and nano-analysis.

EXAMPLE 7

[0204] ZnO nanowires were synthesized using a vapor phase transportprocess via catalyzed epitaxial crystal growth on sapphire (110)substrates. Patterned Au thin film was used as the catalyst for nanowiregrowth. For the nanowire growth, clean (110) sapphire substrates werecoated with 10-35 Angstrom thick gold with or without using TEM grids asshadow masks (micro contact printing of thiols on Au followed byselective etching has also been used to create the Au pattern). An equalamount of ZnO powder and graphite powder were ground and transferred toan alumina boat. The Au-coated sapphire substrates were typically placed0.5-2.5 cm from the center of the boat. The starting materials and thesubstrates were then heated up to 880° C. to 905° C. in an argon flow.Zn vapor was generated by carbothermal reduction of ZnO and transportedto the substrates where ZnO nanowires grew. The growth generally tookplace within 2-10 minutes.

[0205] The nanowires were epitaxially grown on the substrate and formedhighly oriented arrays. Selective nanowire growth can be readilyachieved when patterned Au thin film was used. ZnO nanowires grew onlyin the Au coated area with an excellent selectivity due to the catalyticnature of Au thin layer. The area of these nanowire arrays can bereadily extended to cm². Generally, the diameters of these wires are inthe range of 20 nm to 150 nm while majority of them have diameters of 70nm to 100 nm. The diameter dispersion is due to the inhomogeneous sizesof the Au nanocluster catalysts when the substrate is annealed duringthe growth process. The lengths of these nanowires can be varied between2 μm and 10 μm by adjusting the growth time. This capability ofpatterned nanowire growth allows us to fabricate nanoscale lightemitters on the substrate in a controllable fashion.

[0206] We observed that nearly all the nanowires grew vertically fromthe substrates. This is due to the fact that a good epitaxial interfaceexists between the (0001) plane of the ZnO nanowire and (110) plane ofthe substrate. The ideal a plane (110) of sapphire is two-fold symmetricwhile the ZnO c-plane is six-fold symmetric. They are essentiallyincommensurate with the exception that the ZnO's a -axis and thesapphire's c-axis are related almost exactly by a factor of 4 (mismatchless than 0.08% at room temperature). Such coincidental match up alongthe sapphire [0001] direction, along with a strong tendency of ZnO togrow in the c-orientation as well as the incoherence of interfaces indirections other than sapphire [0001], leads to the unique verticalepitaxial growth configuration. The anisotropy of the sapphire's a planeis critical for growing high quality c-oriented ZnO nanowire arrays.

[0207] SEM images of the nanowire array were obtained. Hexagon endplanes of the nanowires could be clearly identified. This is a strongevidence that these nanowires grow along the <0001> direction and areindeed well-faceted both at the end and side surfaces. The well-facetednature of these nanowires will have important implications when they areused as effective laser media.

[0208] Additional structural characterization of the ZnO nanowires wascarried out using transmission electron microscopy (TEM). A highresolution TEM image of a single-crystalline ZnO nanowire was obtained.A spacing of 2.56±0.05 Angstroms between adjacent lattice planescorresponded to the distance between two (0002) crystal planes, furtherindicating that <0001> is the preferred growth direction for the ZnOnanowires. Significantly, this <0001> preferential nanowire growth onthe sapphire substrate is also reflected in the X-ray diffractionpattern shown in FIG. 40 that was taken on a Siemens Z5000. Only (0001)peaks were observed, indicating excellent overall c-axis alignment ofthese nanowire arrays over a large substrate area.

[0209] Photoluminescence spectra of nanowires were measured using aHe-Cd laser (325 nm) as an excitation source. Strong near-band gap edgeemission at ˜377 nm has been observed. In order to explore the possiblestimulated emission from these oriented nanowires, the power dependentemission was examined. The samples were optically pumped by the fourthharmonic of Nd: YAG laser (266 nm, 3 ns pulse width) at roomtemperature. The pump beam was focused on the nanowires at an incidenceangle of 10 degrees to the symmetric axis of the nanowire. Lightemission was collected in the direction normal to the end surface plane(along the symmetric axis) of the nanowires. Stimulated emission fromthe nanowires was collected in the direction along nanowire's end-planenormal (the symmetric axis) using a monochromator (ISA) combined with aPeltier-cooled CCD (EG&G). All experiments were carried out at roomtemperature. Significantly, in the absence of any fabricated mirrors, weobserved lasing action in these ZnO nanowires.

[0210]FIG. 41 shows the evolution of the emission spectra as pump powerwas increased. At low excitation intensity (below the lasing threshold),the spectrum consists of a single broad spontaneous emission peak (curvea) with a full width at half maximum (FWHM) of approximately 17 nm. Thisspontaneous emission was 140 meV below the band gap (3.37 eV) and isgenerally ascribed to the recombination of excitons throughexciton-exciton collision process where one of the exciton radiativelyrecombines to generate a photon. As the pump power increased, theemission peak narrows due to the preferential amplification offrequencies close to the maximum of the gain spectrum. When theexcitation intensity exceeded the lasing threshold (˜40 kW/cm²), sharppeaks emerge in the emission spectra (curve b and inset). The pump powerfor these spectra were 20, 100, and 150 kW/cm², respectively. The linewidths of these peaks are less than 0.3 nm, which are more than 50 timessmaller than the line width of the spontaneous emission peak below thethreshold. Above the threshold, the integrated emission intensityincreases rapidly with the pump power, as shown in FIG. 42. The narrowline width and the rapid increase of emission intensity indicate thatstimulated emission takes place in these nanowires. The observed singleor multiple sharp peaks (FIG. 41, curve b and inset) represent differentlasing modes at wavelengths between 370 and 400 nm. It was observed thatthe lasing threshold is quite low compared with previously reportedvalues for random lasing (˜300 kW/cm²) in disordered particles or thinfilms. Significantly, these short-wavelength nanowire nanolasers operateat room temperature and the areal density of these nanolasers readilyreaches 1.1×10¹⁰ cm⁻².

[0211] The fact that we observed lasing action in these nanowire arrayswithout any fabricated mirror prompts us to consider thesesingle-crystalline, well-facetted nanowires act as natural resonancecavities to amplify stimulated emission. FIG. 43 schematicallyillustrates a nanolaser 330 fabricated using a multi-faceted (in thisexample, hexagonal) ZnO nanowire 332 grown on a sapphire substrate 334.Note that nanowire 332 is not a heterostructure but a homostructure inthis application, however, it should be appreciated that the presentinvention is capable of heterostructure lasing as well as homostructurelasing. The nanowire acts as a resonant cavity with two naturallyfaceted hexagonal end faces 336, 338 acting as reflecting mirrors. It ispossible that the giant oscillator strength effect, that can occur inhigh quality nanowire crystals with dimensions larger than the excitonBohr radius, but smaller than the optical wavelength, enables theexcitonic stimulated emission in these nanowire arrays. For II-VIsemiconductors, a cleaved edge of the specimen is usually used as amirror. For our nanowires, one end is the epitaxial interface 336between the sapphire substrate 334 and ZnO while the other end is thesharp (0001) plane 338 of the ZnO nanocrystals. Both can serve as goodlaser cavity mirrors considering the refractive indexes for sapphire,ZnO and air are 1.8, 2.45 and 1.0, respectively. Note this is animportant characteristic of this nanowire; namely, that it can beabutted against a waveguide very easily. This natural cavity/waveguideformation in nanowires suggests a simple chemical approach to form ananowire laser cavity without cleavage and etching. In fact, whenmultiple lasing modes were observed for these nanowires (FIG. 41 inset),the observed mode spacing is about 5 nm for ˜5 μm long wires, whichagrees quantitatively well with the calculated spacing between adjacentresonance frequencies v_(F)=c/2nl, where V_(F) is emission mode spacing,c the light speed, n the refractive index and/the resonance cavitylength. Note also that an alternative manner in which a waveguide couldbe formed would be to coat the nanowire with a layer of polymer.

[0212] A laser according to the present invention can have nanowiressupported in a solid polymer or glassy matrix, in solution, or extendingoff of the surface of a substrate. For wires attached to a substrate,the wires can either be disordered, or arranged so that they all pointin the same direction. That direction can be normal to the substratesurface, or can be selected to be any other angle off of the substrate.In addition, even nanowires in a matrix material can be aligned so thatthey form an ordered structure. Note that the present invention includeslasers with the above-described nanowire-composite orientations, as wellas non-laser heterostructures in the same configurations.

[0213] The decay of the luminescence from the ZnO nanowires was studiedusing a frequency-tripled mode-locked Ti: sapphire laser for pulsedexcitation (200 fs pulse length) and a streak camera with ps-resolutionfor detection. Referring to FIG. 44, a good fit (straight line) to theexperimental data (dotted line) recorded at room temperature wasobtained with a biexponential decay model assuming a fast and a slowprocess with time constants of about 70 ps and 350 ps, respectively. Thetime-resolved spectrum was recorded at excitation power of 6.39 mW.Therefore, these lifetime measurements show that the radiativerecombination of the excitons is a superposition of a fast and a slowprocess. The luminescence lifetime is mainly determined by theconcentration of defects, which trap the electrons and/or holes andeventually cause their nonradiative recombination. Although the exactorigin of the luminescence decay remains unclear at this stage, the verylong lifetime measured for these wires demonstrates the high crystalquality achieved with the nanowire growth process. Meantime, it alsoaccounts in part for the low laser threshold reported here.

[0214] In summary, we have demonstrated room-temperature ultravioletlasing in well-oriented vertical ZnO nanowire arrays with a lasingthreshold of 40 kW/cm². The areal density of these nanolasers onsubstrate can readily reach 1.1×10¹⁰ cm 10 ⁻². We anticipate that thelasing wavelength can be tuned into blue region by making alloynanowires of ZnO/CdO. In addition, by creating pn junctions in theseindividual nanowires, one should be able to test the possibility ofmating electron ejection blue lasers from individual nanowires. Suchminiaturized nanowire nano-lasers will find applications innano-photonics and microanalysis.

[0215] From the foregoing it will be appreciated that nanowiresaccording to the present invention can be used as optical cavities.Another way to create an optical cavity would be to create dielectricson the ends of the wires. Additionally, some portions of the nanowirecould have one energy transfer event and others have a different energytransfer event such as in a distributed feedback laser. It will also beappreciated that by capping off the ends of the optical cavity, either alaser or a light amplifier can be realized. Additionally, the cavity canbe a part of the nanowire itself as previously described, the cavitycould be external to the nanowire. In essence, a laser or lightamplifier can be formed from a nanowire, a pumping source, and a cavity,wherein the cavity is part of the nanowire or separate from thenanowire. Furthermore, by using conventional stimulated emissiontechniques, a cavity is not required.

[0216] It will also be appreciated that nanowires according to thepresent invention could be employed as a functional component of aquantum dot laser such as described in U.S. Pat. No. 5,260,957incorporated herein by reference, wherein the quantum dots would beintegrated into the nanowire as described herein and the pumping sourcewould be configured for exciting a population inversion in the quantumdots. Note, however, that a nanowire itself can be pumped for lasingwhere the pumping promotes a population inversion in the nanowire. Thenanowire can be embedded in a polymer matrix as previously described,and can function as an element in a matrix of such lasing devices. Thepumping source can be an optical pumping source, such as a pumpinglaser, or an electrical pumping source having an anode and cathode whichcontact the nanowire either directly or through ohmic contacts. If apumping laser is employed, the wavelength of the pump would preferablybe higher than the nanowire by greater than approximately 10 meV, andmore preferably greater than 100 meV. The nanowire can be placed in acavity or the ends can be formed with reflective faces so that thenanowire functions as a cavity.

[0217] 6.6 Additional Devices.

[0218] From the foregoing, it will be appreciated that a number ofdevices can be fabricated using nanowires and the synthesis methodsdescribed above. Additional specific devices include, but are notlimited to, the following.

[0219] 6.6.1 Field-effect Transistor (FET).

[0220] This is a three terminal device that can be realized using COHNs.The current flowing from the “source” to the “drain” is controlled bythe voltage at the “gate”. The source and drain can be at any two pointsalong the nanowire that contact the nanowire core. The gate contact isapplied to the nanowire sheath at some point between the source anddrain. The gate voltage controls the conductivity of the channel betweenthe source and drain. At least two types of FETs can be fabricated inthis manner. First, a junction FET is fabricated with a reverse bias pnjunction at the gate. In this case the nanowire core is an n-typesemiconductor and the sheath is p-type (or vice versa). Applying areverse bias at the junction can increase the depletion region insidethe core and thus inhibit current flow from the source to drain. Thesecond kind of FET is based on metal-oxide (MOSFET) or metal-insulator(MISFET) contact at the gate. In this case the nanowire sheath is madeof two sub-sheaths. The nanowire core is first covered with a layer ofoxide or insulator and then with a conducting layer. Applying a voltagebetween the conducting sheath and the nanowire core can remove thechannel between source and drain (depletion-mode MOSFET or MISFET) orcreate a channel (enhancement-mode MOSFET or MISFET) if the nanowirecore did not have a conducting channel between the source and drain atzero gate voltage. Additionally, a three-terminal device such as, butnot limited to, those depicted in FIG. 34 and FIG. 35 could be utilizedwith two terminals acting as source and drain and the third acting as agate. In particular, it is possible to create a MOSFET structure in athree-terminal device in which an insulating heterojunction is placedbetween the source-drain path and the gate-electrode.

[0221] 6.6.2 Infrared Detector.

[0222] An infrared (IR) detector can be made using nanowires made ofsemiconducting material having a bandgap within the infrared wavelengths(1-20 microns). The detector is preferably a two terminal device, withtwo connections at the two ends of the nanowire. Presence of lightchanges the conductivity of the nanowire, which is measured using anapplied bias between the two terminals (photoconductor); or the lightcreates a voltage across the nanowire with no external biasing circuit(photovoltaic operation). Photovoltaic operation requires an internalelectric field in the nanowire. This can be realized using p-n junctionalong the nanowire or metal/semiconductor junction at the contacts tothe two extremes of nanowire. When the nanowire diameter is smaller thanthe electron deBroglie wavelength of electrons, quantum confinementeffect will change the effective bandgap of material and the region ofsensitivity to IR radiation. A second type of IR detector can befabricated using LOHNs. Similar to quantum well infrared intersubbandphotodetectors, a series of heterostructures along the direction ofnanowire create quantized electronic states inside conduction band orvalence band of the material. Optical absorption between theseelectronic states can be tailored to be at any IR wavelength (1-20microns), not limited by the bandgap of the material. Again contacts atthe two ends of nanowire will allow realization of a photoconductive orphotovoltaic IR detector.

[0223] 6.6.3 Single Electron Infrared Detector.

[0224] This device is similar to the previous LOHN-based infrareddetector. The only difference is that the heterostructure layerparameters (length, nanowire diameter, composition) are chosen so thatcreation of a free electron inside that particular layer changes theelectrostatic energy by an amount so that no other electrons can betransported across this layer until the free electron leaves (coulombblockade). This enables detecting single electron (and thus singlephoton) events.

[0225] 6.6.4 Resonant Tunneling Diode.

[0226] This is a two terminal device made of LOHNs. The basic idea isthat a LOHN is divided into five segments (emitter, barrier 1, well,barrier 2, collector). The well layer is short enough so that electronicenergy states are quantized. The band structure of the barrier layer ischosen so that electron wavefunction is evanescent but the electrontransmission probability across the layer is non-zero. Under a bias,electrons are emitted from the emitter contact to the collector contact.At a specific bias, so that the energy of incident electrons fromemitter corresponds to the quantized energy levels in the well,transmission across the whole structure is enhanced (resonant tunneling)which gives rise to negative differential resistance in the devicecurrent-voltage characteristics, and can be utilized by way of example,to make high speed oscillators or logic circuits.

[0227] 6.6.5 Light Emitting Diode.

[0228] A single nanowire light emitting diode can be made of pn junctionalong the nanowire. A heterostructure near the depletion region (whereelectrons and holes recombine), can be used to make more efficient LEDsby confining the carriers. In order to make LEDs with an array ofnanowires, it is important to incorporate an appropriate filling(polymer, etc.) with low absorption and scattering losses. The finaldevice will be composed of two electrodes with nanowire composite inbetween.

[0229] 6.6.6 Electrically Pumped Laser.

[0230] This is made of nanowire composite LEDs plus an optical cavity.The optical cavity can be made of dielectric mirrors in verticalconfiguration (along with contacts to the two sides of the nanowirecomposite material) or in a horizontal configuration (similar todistributed Bragg reflector lasers).

[0231] 6.6.7 Optical Waveguide/Interconnect.

[0232] In this configuration, nanowires are used either as a part ofcomposite material (nanowire plus filling material) or light is guidedinside the nanowire itself. In the latter case the main parameters inthe design are the optical loss along the wire and the number of lateralmodes. Typical design is based on COHNs, where the indices of the coreand sheath layers are chosen to achieve a specific number of guidedmodes or group dispersion values (this is similar to the design ofsilica-based optical fibers). For the former case (nanowire compositematerial), it can be treated as a new engineered material andconventional methods to fabricate waveguides (ridge type, slab layer,etc.) can be used.

[0233] 6.6.8 Optical Coupler/Modulators/Switch.

[0234] In the case of nanowire composite materials, basically we willhave a filling material (e.g. a polymer) whose electro optical, thermooptical or magneto optical properties are modified by the incorporationof nanowire arrays. One can choose different materials and various wirediameters to tune the required properties (increase electro-opticcoefficient, incorporate optical gain with nanowire pn junctions, etc.).Once the nanowire composite material is fabricated, it can be treated asa novel thin film material and conventional techniques to realizewaveguide switches, modulators, couplers, etc. can be used. The mainadvantage is that the rich properties of passive and activeheterostructure nanowires is combined with simple processing of polymerbased thin film devices.

[0235] 6.6.9 Electromechanical/Thermomechanical Devices.

[0236] Nanowires made of piezoelectric or piezoresistive materials couldbe used as electromechanical sensors. Under uniaxial strain, in thedirection along the wire (longitudinal), a piezoelectric (e.g., AIN,ZnO) nanowire can produce a voltage signal, whereas a piezoresistivenanowire (e.g., Si) will produce a change in resistance, which can bemeasured by passing a current. When these are formed into a polymermatrix composite one can essentially get a flexible/conformal materialthat can be used to detect uniaxial strain. The piezoelectric device canbe used to generate uniaxial motion as well. If nanowireheterostructures are formed in a way that a single crystal nanowire iscoated on one side with another material (e.g., a partial COHN asdescribed herein), then it could be used to generate bending motion as abimorph. For example, if the two materials have different thermalexpansion coefficients, they could be used for detecting temperature andanything that changes temperature (radiation absorption, electricaldissipation, and for forth). In addition, by changing the temperaturethis device could be used for thermal actuation. A nanowire-basedbimorph can also be used to detect any strain perpendicular to thedirection of the nanowire axis.

[0237] 6.6.10 Chemical Sensing Devices.

[0238] While nanowires can behave as chemical sensors, they can also beused for chemical logic. For example, consider a LOHN, which hassegments of materials A, B, C. Supposing that material A becomesconductive when it adsorbs chemical A′, while similarly B becomesconductive when it adsorbs material B′. Now only in the presence ofchemicals A′, B′ and C′, will high conductivity exist in the nanowire.This is a sort of chemical logic, i.e. A′ and B′ and C′=1 such that A′and B′ and not C′=0 etc. If you put these structures in series/parallelnetworks, you could then generate AND and OR logic. One could of courseextend this to biological sensing. In fact, it will be much easier forbiological sensing, since bio-receptors are highly specific.

[0239] 7. Conclusion.

[0240] From the foregoing discussion it can be seen that whensemiconductors are confined to 2, 1 or 0 dimensional structures in thesize range of less than approximately 200 nm, and preferably in therange of approximately 5 nm to 50 nm, their properties can bemanipulated in novel ways. The methods described herein along with otherchemical synthesis techniques can be used to grow nanowires andassociated heterostructures. These structures include coaxialheterostructure nanowires (COHNs) and longitudinal heterostructurenanowires (LOHNs) and combinations thereof. COHNs allow modulationdoping such that nanowires with extremely high charge carrier mobilitycan be obtained, while LOHNs allow bandgap engineering in 1-D, which canlead to multiple quantum dots or pn junctions integrated within a singlenanowire. Engineering the band structure near the Fermi level will alsoallow tailoring of their thermoelectric properties. 1-D confinement hasa strong influence on phonon spectra and lifetimes, which coulddramatically change their thermal properties. In addition, nanowireheterostructures offer the promising prospects of integratingpiezoelectric nanostructures with semiconducting nanowires, resulting innanoelectromechanical transducers. Also, the elastic boundary conditionsin COHNs and LOHNs enable dislocation-free interlaces that are notstable in 2-D (quantum wells and heterostructures) or thin film form,while also providing access to new stable phases that are metastable inbulk or thin film forms.

[0241] COHNs and LOHNs also lend themselves to the development of energyconversion devices, including thermoelectric refrigerators or powergenerators, light emitting devices, and nanoelectromechanicaltransducers. The active material in these devices comprises compositesmade of nanowire arrays preferably embedded in a polymer matrix, suchthat they can be patterned and integrated into microsystems.Semiconducting nanowires with diameters in the 5 nm to 10 nm rangeprovide the unique opportunity to develop thermoelectric refrigeratorsand power generators with performances comparable to or better thanthose based on gas or vapor. Such solid-state devices could haveenormous impact on energy utilization technology as well as on theenvironment. Use of nanowires containing integrated quantum dots can allefficient and size-tunable optoelectronic conversion.

[0242] Furthermore, embedding such nanowires in a polymer matrix willproduce a light emitting flexible medium with much lower effective indexthan semiconductors, which will enable efficient coupling with opticalfibers and thereby dramatically improve external quantum efficiencies.When combined with single electronics, such quantum dot nanowires offerthe possibility of single photonic devices that can significantly impactinformation storage and processing. Nanowire-based piezoelectrictransducers will lead to high quality factor and high resonant frequencydevices that can be used for applications ranging from molecular sensingand nano-actuators to high-frequency signal processors. Finally,single-crystalline nanowires with naturally faceted end facesfunctioning as mirrors can be used for nanolaser devices.

[0243] LOHNs containing heterostructures along the nanowire, length canalso be designed to have very interesting properties, including, but notlimited to: (a) pn or pnp or various other junctions that could be usedfor photonic devices, (b) multiple quantum dots with size-tunableoptical absorption/emission and single electron tunneling properties,thus leading to single photonic devices; (c) nanowire superlattices withhigh electron mobility and reduced phonon transport, and thereby usefulfor thermoelectric devices; and (d) piezoelectric and electronicheterodevices for nanoelectromechanical transduction. Note also that theelastic boundary conditions of nanowire heteroepitaxial growth offer thepossibility to create dislocation free interfaces in the superlatticenanowires that are not stable in the conventional 2-D configurationachieved by epitaxial film growth on planar substrates. On the otherhand, there are cases where dislocations are desirable as well, and thepresent invention allows for defect control.

[0244] Although much effort in the past has been focused on synthesizingand studying zero-dimensional (0-D) (quantum dots) and 2-D (quantumwells and heterostructures) nanostructures, semiconducting nanowiresother than carbon nanotubes have received relatively little attention.Note, however, that compared to quantum dots, nanowires that areapproximately 1 μm to approximately 10 μm long provide the uniqueopportunity of integrating nanostructures with photolithographicallymicrofabricated features that are generally ≈1 μm. In addition,nanowires also allow further confinement over 2-D structures that havebeen extensively studied and utilized in the past. Because of thesefavorable attributes, various other types of devices based on nanowireheterostructures can be designed and fabricated as well, including, butnot limited to: (i) high efficiency thermoelectric refrigerators orpower generators; (ii) tunable light emitting diodes; (iii)piezoelectric nanomechanical sensors and actuators. The use of nanowireheterostructures in these devices is critical, since they would eitherdramatically improve conversion efficiencies or open up new ways ofconversion, as discussed in detail later. These simple devices also formthe foundation for more sophisticated devices.

[0245] It will be appreciated that various configurations can beachieved using the foregoing inventive structures, some of which havebeen previously described. By way of further example, and not oflimitation, these configurations can include single and multiplejunction LOHNs, single and multiple junction COHNs, combinations of LOHNand COHN structures, two-terminal configurations, N>2 terminalconfigurations, combinations of heterostructures and homostructures,homostructures with one or more electrodes (which would also be anoverall heterostructure), heterostructures with one or more electrodes,homostructures with insulators, heterostructures with insulators, andthe like. It will also be appreciated that the interface between ananowire and a terminal constitutes a heterojunction. A variety ofdevices can be fabricated using these structures and configurations,including, but not limited to, phonon bandgap devices, quantum dots thatconfine electrons in specific areas, thermoelectric devices (e.g., solidstate refrigerators and engines), photonic devices (e.g., nanolasers),nanoelectromechanical (MEM) devices (electromechanical actuators andsensors), energy conversion devices of various forms including forexample, light to mechanical energy or thermal energy to light, andother devices.

[0246] Although the description above contains many details, theseshould not be construed as limiting the scope of the invention but asmerely providing illustrations of some of the presently preferredembodiments of this invention. Therefore, it will be appreciated thatthe scope of the present invention fully encompasses other embodimentswhich may become obvious to those skilled in the art, and that the scopeof the present invention is accordingly to be limited by nothing otherthan the appended claims, in which reference to an element in thesingular is not intended to mean “one and only one” unless explicitly sostated, but rather “one or more.” All structural, chemical, andfunctional equivalents to the elements of the above-described preferredembodiment that are known to those of ordinary skill in the art areexpressly incorporated herein by reference and are intended to beencompassed by the present claims. Moreover, it is not necessary for adevice or method to address each and every problem sought to be solvedby the present invention, for it to be encompassed by the presentclaims. Furthermore, no element, component, or method step in thepresent disclosure is intended to be dedicated to the public regardlessof whether the element, component, or method step is explicitly recitedin the claims. No claim element herein is to be construed under theprovisions of 35 U.S.C. 112, sixth paragraph, unless the element isexpressly recited using the phrase “means for.”

What is claimed is:
 1. A nanowire, comprising: a first segment of afirst material; and a second segment of a second material joined to saidfirst segment; wherein at least one of said segments has a substantiallyuniform diameter of less than approximately 200 nm; and wherein saidnanowire is selected from a population of nanowires having asubstantially monodisperse distribution of diameters.
 2. A nanowire,comprising: a first segment of a first material; and a second segment ofa second material joined to said first segment; wherein at least one ofsaid segments has a substantially uniform diameter of less thanapproximately 200 nm; and wherein said nanowire is selected from apopulation of nanowires having a substantially monodisperse distributionof lengths.
 3. A nanowire, comprising: a first segment of a firstmaterial; and a second segment of a second material joined to said firstsegment; said nanowire displaying characteristics selected from thegroup consisting essentially of electronic properties, opticalproperties, physical properties, magnetic properties and chemicalproperties that are modified relative to the bulk characteristics ofsaid first and second materials by quantum confinement effects.
 4. Ananowire, comprising: a first segment of a first material; and a secondsegment of a second material joined to said first segment; said nanowirehaving at least one electronic property that varies as a function ofdiameter of said nanowire.
 5. A nanowire as recited in claim 4, whereinsaid at electronic property comprises band-gap energy.
 6. A nanowire,comprising: a first segment of a substantially crystalline material; anda second segment of a substantially crystalline material joined to saidfirst segment; wherein at least one of said segments has a substantiallyuniform diameter of less than approximately 200 nm.
 7. A nanowire asrecited in claim 6, wherein each of said first and said second segmentscomprises a doped semiconductor material.
 8. A nanowire as recited inclaim 7, wherein said doped semiconductor material is selected from thegroup consisting essentially of a group III-V semiconductor, a groupII-VI semiconductor, a group II-IV semiconductor, and tertiaries andquaternaries thereof.
 9. A nanowire as recited in claim 6, wherein eachof said first and second segments exhibits the electricalcharacteristics of a homogeneously doped semiconductor.
 10. A nanowire,comprising: a first segment of a substantially crystalline material; anda second segment of a compositionally different material joined to saidfirst segment; wherein at least one of said segments has a substantiallyuniform diameter of less than approximately 200 nm.
 11. A nanowire asrecited in claim 10, wherein said second segment comprises asubstantially crystalline material.
 12. A nanowire, comprising: a firstsegment of semiconductor material; and a second segment of semiconductormaterial joined to said first segment; wherein at least one of saidsegments has a substantially uniform diameter of less than approximately200 nm.
 13. A nanowire as recited in claim 12, wherein each of saidfirst and said second segments comprise a doped semiconductor material.14. A nanowire as recited in claim 13, wherein said doped semiconductormaterial is selected from the group consisting essentially of a groupIII-V semiconductor, a group II-VI semiconductor, a group II-IVsemiconductor, and tertiaries and quaternaries thereof.
 15. A nanowireas recited in claim 12, wherein each of said first and second segmentsexhibits the electrical characteristics of a homogeneously dopedsemiconductor.
 16. A nanowire, comprising: a first segment of dopedsemiconductor material; and a second segment of doped semiconductormaterial joined to said first segment; wherein at least one of saidsegments has a substantially uniform diameter of less than approximately200 nm.
 17. A nanowire as recited in claim 16, wherein said dopedsemiconductor material is selected from the group consisting essentiallyof a group III-V semiconductor, a group II-VI semiconductor, a groupII-IV semiconductor, and tertiaries and quaternaries thereof.
 18. Ananowire as recited in claim 16, wherein each of said first and secondsegments exhibits the electrical characteristics of a homogeneouslydoped semiconductor.
 19. A nanowire, comprising: a first segment of asubstantially crystalline material; and a second segment of acompositionally different material joined to said first segment; whereinsaid nanowire transitions from said first segment to said second segmentover a distance ranging from approximately one atomic layer toapproximately 20 nm; and wherein at least one of said segments has asubstantially uniform diameter of less than approximately 200 nm.
 20. Ananowire as recited in claim 19, wherein said second segment comprises asubstantially crystalline material.
 21. A nanowire, comprising: a firstsegment of a substantially crystalline material; and a second segment ofa substantially crystalline material joined to said first segment;wherein said nanowire transitions from said first segment to said secondsegment over a distance ranging from approximately one atomic layer toapproximately 20 nm; and wherein at least one of said segments has asubstantially uniform diameter of less than approximately 200 nm.
 22. Ananowire as recited in claim 21, wherein each of said first and saidsecond segments comprises a semiconductor material.
 23. A nanowire asrecited in claim 21, wherein each of said first and said second segmentscomprises a doped semiconductor material.
 24. A nanowire as recited inclaim 23, wherein said doped semiconductor material is selected from thegroup consisting essentially of a group III-V semiconductor, a groupII-VI semiconductor, a group II-IV semiconductor, and tertiaries andquaternaries thereof.
 25. A nanowire as recited in claim 21, whereineach of said first and second segments exhibits the electricalcharacteristics of a homogeneously doped semiconductor.
 26. A nanowire,comprising: a first segment of semiconductor material; and a secondsegment of semiconductor material joined to said first segment; whereinsaid nanowire transitions from said first segment to said second segmentover a distance ranging from approximately one atomic layer toapproximately 20 nm; and wherein at least one of said segments has asubstantially uniform diameter of less than approximately 200 nm.
 27. Ananowire as recited in claim 26, wherein each of said first and saidsecond segments comprises a doped semiconductor material.
 28. A nanowireas recited in claim 27, wherein said doped semiconductor material isselected from the group consisting essentially of a group III-Vsemiconductor, a group II-VI semiconductor, a group II-IV semiconductor,and tertiaries and quaternaries thereof.
 29. A nanowire as recited inclaim 26, wherein each of said first and second segments exhibits theelectrical characteristics of a homogeneously doped semiconductor.
 30. Ananowire, comprising: a first segment of doped semiconductor material;and a second segment of doped semiconductor material joined to saidfirst segment; wherein said nanowire transitions from said first segmentto said second segment over a distance ranging from approximately oneatomic layer to approximately 20 nm; and wherein at least one of saidsegments has a substantially uniform diameter of less than approximately200 nm.
 31. A nanowire as recited in claim 30, wherein said dopedsemiconductor material is selected from the group consisting essentiallyof a group III-V semiconductor, a group II-VI semiconductor, a groupII-IV semiconductor, and tertiaries and quaternaries thereof.
 32. Ananowire as recited in claim 30, wherein each of said first and secondsegments exhibits the electrical characteristics of a homogeneouslydoped semiconductor.
 33. A nanowire, comprising: a first segment of asubstantially crystalline material; and a second segment of acompositionally different material joined to said first segment; whereinsaid nanowire transitions from said first segment to said second segmentover a distance ranging from approximately one atomic layer toapproximately 20 nm; wherein transition from said first segment to saidsecond segment begins at a point toward said second segment where thecomposition of said first segment has decreased to approximately 99% ofthe composition of said first segment at the center of said firstsegment; and wherein at least one of said segments has a substantiallyuniform diameter of less than approximately 200 nm.
 34. A nanowire asrecited in claim 33, wherein said second segment comprises asubstantially crystalline material.
 35. A nanowire, comprising: a firstsegment of a substantially crystalline material; and a second segment ofa substantially crystalline material joined to said first segment;wherein said nanowire transitions from said first segment to said secondsegment over a distance ranging from approximately one atomic layer toapproximately 20 nm; wherein transition from said first segment to saidsecond segment begins at a point toward said second segment where thecomposition of said first segment has decreased to approximately 99% ofthe composition of said first segment at the center of said firstsegment; wherein at least one of said segments has a diameter of lessthan approximately 200 nm; and wherein the diameter of said at least oneof said segments having a diameter of less than approximately 200 nmdoes not vary by more than approximately 10% over the length of saidsegment.
 36. A nanowire as recited in claim 35, wherein each of saidfirst and said second segments comprises a semiconductor material.
 37. Ananowire as recited in claim 35, wherein each of said first and saidsecond segments comprises a doped semiconductor material.
 38. A nanowireas recited in claim 37, wherein said doped semiconductor material isselected from the group consisting essentially of a group II-Vsemiconductor, a group II-VI semiconductor, a group II-IV semiconductor,and tertiaries and quaternaries thereof.
 39. A nanowire as recited inclaim 35, wherein each of said first and second segments exhibits theelectrical characteristics of a homogeneously doped semiconductor.
 40. Ananowire, comprising: a first segment of semiconductor material; and asecond segment of semiconductor material joined to said first segment;wherein said nanowire transitions from said first segment to said secondsegment over a distance ranging from approximately one atomic layer toapproximately 20 nm; wherein transition from said first segment to saidsecond segment begins at a point toward said second segment where thecomposition of said first segment has decreased to approximately 99% ofthe composition of said first segment at the center of said firstsegment; wherein at least one of said segments has a diameter of lessthan approximately 200 nm; and wherein the diameter of said at least oneof said segments having a diameter of less than approximately 200 nmdoes not vary by more than approximately 10% over the length of saidsegment.
 41. A nanowire as recited in claim 40, wherein each of saidfirst and said second segments comprises a doped semiconductor material.42. A nanowire as recited in claim 41, wherein said doped semiconductormaterial is selected from the group consisting essentially of a groupIII-V semiconductor, a group II-VI semiconductor, a group II-IVsemiconductor, and tertiaries and quaternaries thereof.
 43. A nanowireas recited in claim 40, wherein each of said first and second segmentsexhibits the electrical characteristics of a homogeneously dopedsemiconductor.
 44. A nanowire, comprising: a first segment of dopedsemiconductor material; and a second segment of doped semiconductormaterial joined to said first segment; wherein said nanowire transitionsfrom said first segment to said second segment over a distance rangingfrom approximately one atomic layer to approximately 20 nm; whereintransition from said first segment to said second segment begins at apoint toward said second segment where the composition of said firstsegment has decreased to approximately 99% of the composition of thefirst segment at the center of said first segment; wherein at least oneof said segments has a diameter of less than approximately 200 nm; andwherein the diameter of said at least one of said segments having adiameter of less than approximately 200 nm does not vary by more thanapproximately 10% over the length of said segment.
 45. A nanowire asrecited in claim 44, wherein said doped semiconductor material isselected from the group consisting essentially of a group II-Vsemiconductor, a group II-VI semiconductor, a group II-IV semiconductor,and tertiaries and quaternaries thereof.
 46. A nanowire as recited inclaim 44, wherein each of said first and second segments exhibits theelectrical characteristics of a homogeneously doped semiconductor.
 47. Ananowire as recited in claim 1, 2, 3 or 4, wherein at least one of saidmaterials comprises a substantially crystalline material.
 48. A nanowireas recited in claim 1, 2, 3, 4, 6, 12, 16, 21, 26, 30, 35, 40 or 44,wherein said first and second materials are compositionally differentmaterials.
 49. A nanowire as recited in claim 1, 2, 3, 4, 6, 10, 12, 16,19, 21, 26, 30, 35, 40 or 44, wherein at least one of said segmentscomprises a substantially monocrystalline material.
 50. A nanowire asrecited in claim 1, 2, 3, 4, 6, 10, 12 or 16, wherein said nanowiretransitions from said first segment to said second segment over adistance ranging from approximately one atomic layer to approximately100 nm.
 51. A nanowire as recited in claim 50, wherein said transitionoccurs over a region that is substantially defect free.
 52. A nanowireas recited in claim 50, wherein said transition occurs over a regionthat is substantially crystalline.
 53. A nanowire as recited in claim50, wherein said transition occurs over a region that is substantiallymonocrystalline.
 54. A nanowire as recited in claim 50, whereintransition from said first segment to said second segment begins at apoint toward said second segment where the composition of said firstsegment has decreased to approximately 99% of the composition of saidfirst segment at the center of said first segment.
 55. A nanowire asrecited in claim 1, 2, 3, 4, 5, 6, 10, 12 or 16, wherein said nanowiretransitions from said first segment to said second segment over adistance ranging from approximately one atomic layer to approximately 20nm.
 56. A nanowire as recited in claim 55, wherein said transitionoccurs over a region that is substantially defect free.
 57. A nanowireas recited in claim 55, wherein said transition occurs over a regionthat is substantially crystalline.
 58. A nanowire as recited in claim55, wherein said transition occurs over a region that is substantiallymonocrystalline.
 59. A nanowire as recited in claim 55, whereintransition from said first segment to said second segment begins at apoint toward said second segment where the composition of said firstsegment has decreased to approximately 99% of the composition of saidfirst segment at the center of said first segment.
 60. A nanowire asrecited in claim 1, 2, 3, 4, 6, 10, 19, 21, 33 or 35, wherein at leastone of said segments comprises a semiconductor material.
 61. A nanowireas recited in claim 1, 2, 3, 4, 6, 10, 12, 19, 21, 26, 33, 35 or 40,wherein at least one of said segments comprises a doped semiconductormaterial.
 62. A nanowire as recited in claim 1, 2, 3, 4, 6, 20, 12, 16,19, 21, 26, 30, 33, 35, 40 or 44, wherein at least one of said segmentsexhibits the electrical characteristics of a homogeneously dopedsemiconductor.
 63. A nanowire as recited in claim 1, 2, 6, 10, 12, 16,19, 21, 26, 30, 33, 35, 40 or 44, wherein said at least one of saidsegments having a diameter of less than approximately 200 nm has adiameter ranging from approximately 5 nm to approximately 50 nm.
 64. Ananowire as recited in claim 1, 2, 6, 10, 12, 16, 19, 21, 26, 30, 33,35, 40 or 44, wherein the diameter of said at least one of said segmentshaving a diameter of less than approximately 200 nm does not vary bymore than approximately 50% over the length of said segment.
 65. Ananowire as recited in claim 1, 2, 6, 10, 12, 16, 19, 21, 26, 30, 33,35, 40 or 44, wherein the diameter of said at least one of said segmentshaving a diameter of less than approximately 200 nm does not vary bymore than approximately 10% over the length of said segment.
 66. Ananowire as recited in claim 1, 2, 3, 4, 6, 10, 12, 16, 19, 21, 26, 30,33, 35, 40 or 44, wherein said second segment is longitudinally adjacentsaid first segment.
 67. A nanowire as recited in claim 1, 2, 3, 4, 6,10, 12, 16, 19, 21, 26, 30, 33, 35, 40 or 44, wherein said secondsegment is coaxially adjacent said first segment.
 68. A nanowire asrecited in claim 1, 2, 3, 4, 6, 10, 12, 16, 19, 21, 26, 30, 33, 35, 40or 44, wherein said first segment comprises a substantiallymonocrystalline material.
 69. A nanowire as recited in claim 1, 2, 3, 4,6, 10, 12, 16, 19, 21, 26, 30, 33, 35, 40 or 44, wherein said secondsegment comprises a substantially monocrystalline material.
 70. Ananowire as recited in claim 1, 2, 3, 4, 6, 10, 12, 16, 19, 21, 26, 30,33, 35, 40 or 44, wherein said first and second segments form a p-njunction.
 71. A nanowire as recited in claim 70, wherein said nanowirecomprises a semiconductor device.
 72. A nanowire as recited in claim 1,2, 3, 4, 6, 10, 12, 16, 19, 21, 26, 30, 33, 35, 40 or 44, wherein saidfirst and second segments form a p-i junction.
 73. A nanowire as recitedin claim 72, wherein said nanowire comprises a semiconductor device. 74.A nanowire as recited in claim 1, 2, 3, 4, 6, 10, 12, 16, 19, 21, 26,30, 33, 35, 40 or 44, wherein said first and second segments form a i-njunction.
 75. A nanowire as recited in claim 74, wherein said nanowirecomprises a semiconductor device.
 76. A nanowire as recited in claim 1,2, 3, 4, 6, 10, 12, 16, 19, 21, 26, 30, 33, 35, 40 or 44, furthercomprising an electrode electrically coupled to at least one of saidsegments.
 77. A nanowire as recited in claim 1, 2, 3, 4, 6, 10, 12, 16,19, 21, 26, 30, 33, 35, 40 or 44, wherein at least one of said segmentscomprises a material selected from the group of elements consistingessentially of group II, group III, group IV, group V, group VIelements, and tertiaries and quaternaries thereof.
 78. A nanowire asrecited in claim 1, 2, 3, 4, 6, 10, 12, 16, 19, 21, 26, 30, 33, 35, 40or 44, wherein at least one of said segments is embedded in a polymermatrix.
 79. A nanowire as recited in claim 1, 2, 3, 4, 6, 10, 12, 16,19, 21, 26, 30, 33, 35, 40 or 44, wherein at least a portion of at leastone of said segments is covered by a sheath.
 80. A nanowire as recitedin claim 79, wherein said sheath comprises an amorphous material.
 81. Ananowire as recited in claim 79, wherein said sheath comprises asubstantially crystalline material.
 82. A nanowire as recited in claim81, wherein said substantially crystalline material is substantiallymonocrystalline.
 83. A nanowire as recited in claim 1, 2, 3, 4, 6, 10,12, 16, 19, 21, 26, 30, 33, 35, 40 or 44: wherein said nanowire is afunctional component of a device selected from the group of devicesconsisting essentially of phonon bandgap devices, quantum dot devices,thermoelectric devices, photonic devices, nanoelectromechanicalactuators, nanoelectromechanical sensors), field-effect transistors,infrared detectors, resonant tunneling diodes, single electrontransistors, infrared detectors, magnetic sensors, light emittingdevices, optical modulators, optical detectors, optical waveguides,optical couplers, optical switches, and lasers.
 84. A nanowire asrecited in claim 1, 2, 3, 4, 6,10, 12, 16, 19, 21, 26, 30, 33, 35, 40 or44, wherein said nanowire is an element of an array of nanowires.
 85. Ananowire as recited in claim 84, wherein said array comprises anoriented array.
 86. A nanowire as recited in claim 84, wherein each ofsaid nanowires in said array is oriented at an angle substantiallynormal to a substrate.
 87. A nanowire as recited in claim 84, whereineach of said nanowires in said array is oriented at an angle that is notnormal to a substrate.
 88. A nanowire as recited in claim 1, 2, 3, 4, 6,10, 12, 16, 19, 21, 26, 30, 33, 35, 40 or 44, electrically coupled to asecond nanowire wherein a junction is formed.
 89. A nanowire as recitedin 88, wherein said nanowire is in ohmic contact with said secondnanowire.
 90. A nanowire as recited in claim 88, wherein said nanowireis inductively coupled to said second nanowire.
 91. A nanowire asrecited in claim 88, wherein said nanowire forms a tunneling junctionwith said second nanowire.
 92. A nanowire as recited in claim 88,wherein said junction has a substantially linear voltage-currentrelationship.
 93. A nanowire as recited in claim 88, wherein saidjunction has a substantially non-linear voltage-current relationship.94. A nanowire as recited in claim 88, wherein said junction has asubstantially step function voltage-current relationship.
 95. A nanowirecollection, comprising: a plurality of a nanowires as recited in any ofclaims 1, 2, 3, 4, 6, 10, 12, 16, 19, 21, 26, 30, 33, 35, 40 or
 44. 96.A nanowire collection as recited in claim 95, wherein said collectioncomprises greater than approximately 100 nanowires.
 97. A nanowirecollection as recited in claim 95, wherein said collection comprisesgreater than approximately 1000 nanowires.
 98. A nanowire collection asrecited in claim 95, wherein greater than 80% of the members of saidcollection comprise substantially the same heterostructure.
 99. Ananowire collection as recited in claim 95, wherein substantially all ofthe members of said collection exhibit substantially the sameheterostructure.
 100. A nanowire collection as recited in claim 95,wherein the members of said collection comprise at least two differentspecies of nanowire.
 101. A nanowire collection as recited in claim 95,wherein the members of said collection comprise at least ten differentspecies of nanowire.
 102. A nanowire collection as recited in claim 95,wherein said collection is suspended in a fluid.
 103. A nanowirecollection as recited in claim 95, wherein said collection is suspendedby a material selected from the group consisting essentially of aliquid, a glass, a gel, and a gas.
 104. A nanowire collection as recitedin claim 95, wherein said collection is suspended or embedded in amatrix.
 105. A nanowire collection as recited in claim 95, wherein oneor more members of said collection is electrically coupled to one ormore other members of said collection.
 106. A nanowire collection asrecited in claim 105, wherein one or more members of said collection isin ohmic contact with one or more other members of said collection. 107.A nanowire collection as recited in claim 105, wherein one or moremembers of said collection is inductively coupled with one or more othermembers of said collection.
 108. A nanowire collection as recited inclaim 105, wherein one or more members of said collection forms atunneling junction with one or more other members of said collection.109. A nanowire collection as recited in claim 105, wherein saidelectric coupling has a substantially non-linear voltage-currentrelationship.
 110. A nanowire collection as recited in claim 105,wherein said electric coupling has a substantially linearvoltage-current relationship.
 111. A nanowire collection as recited inclaim 105, wherein said electric coupling has a substantially stepfunction voltage-current relationship.
 112. A nanowire collection asrecited in claim 95, wherein said collection has a substantiallymonodisperse distribution of nanowire diameters.
 113. A collection ofnanowires as recited in claim 95, wherein said collection has asubstantially monodisperse distribution of nanowire lengths.
 114. Ananowire, comprising: a first segment of a first material; a secondsegment of a second material joined to said first segment; and a thirdsegment of a third material joined to at least one of said first andsecond segments; wherein at least one of said segments has asubstantially uniform diameter of less than approximately 200 nm;wherein at least two of said materials comprise compositionallydifferent materials; and wherein at least two of said segments areadjacent.
 115. A nanowire as recited in claim 114, wherein the diameterof said at least one of said segments having a diameter of less thanapproximately 200 nm does not vary by more than approximately 10% overthe length of said segment.
 116. A nanowire as recited in claim 114,wherein said nanowire transitions from at least one of said segments toan adjacent segment over a distance ranging from approximately oneatomic layer to approximately 20 nm.
 117. A nanowire as recited in claim116, wherein said transition begins at a point moving from said at leastone of said segments toward said adjacent segment where the compositionof said at least one of said segments has decreased to approximately 99%of the composition of that segment at its center.
 118. A nanowire asrecited in claim 114, wherein at least two of said segments arelongitudinally adjacent.
 119. A nanowire as recited in claim 114:wherein said second segment is longitudinally adjacent said firstsegment; and wherein said third segment is longitudinally adjacent saidsecond segment.
 120. A nanowire as recited in claim 114, wherein atleast two of said segments are coaxially adjacent.
 121. A nanowire asrecited in claim 114, wherein at least one of said materials comprises asubstantially crystalline material.
 122. A nanowire as recited in claim114, wherein said substantially crystalline material is substantiallymonocrystalline.
 123. A nanowire as recited in claim 114, wherein atleast one of said segments comprises a semiconductor material.
 124. Ananowire as recited in claim 114, wherein at least one of said segmentscomprises a doped semiconductor material.
 125. A nanowire as recited inclaim 114, wherein at least one of said segments exhibits the electricalcharacteristics of a homogeneously doped semiconductor.
 126. A nanowireas recited in claim 114, wherein said at least one of said segmentshaving a diameter of less than approximately 200 nm has a diameterranging from approximately 5 nm to approximately 50 nm.
 127. A nanowireas recited in claim 114, wherein at least two of said segments form ap-n junction.
 128. A nanowire as recited in claim 114, wherein at leasttwo of said segments form a p-i junction.
 129. A nanowire as recited inclaim 114, wherein at least two of said segments form a i-n junction.130. A nanowire as recited in claim 114, wherein said segments formp-n-p junctions.
 131. A nanowire as recited in claim 114, wherein saidsegments a n-p-n junctions.
 132. A nanowire as recited in claim 114,wherein said segments form p-i-n junctions.
 133. A nanowire as recitedin claim 114, wherein said segments form p-i-p junctions.
 134. Ananowire as recited in claim 127, 128, 129, 130, 131, 132 or 133,wherein said nanowire comprises a semiconductor device.
 135. A nanowireas recited in claim 114, further comprising an electrode electricallycoupled to at least one of said segments.
 136. A nanowire as recited inclaim 114, wherein at least one of said segments comprises a materialselected from the group of elements consisting essentially of group II,group III, group IV, group V, and group VI elements, and tertiaries andquaternaries thereof.
 137. A nanowire as recited in claim 114, whereinat least one of said segments is embedded in a polymer matrix.
 138. Ananowire as recited in claim 114, wherein at least a portion of at leastone of said segments is covered by a sheath.
 139. A nanowire as recitedin claim 138, wherein said sheath comprises an amorphous material. 140.A nanowire as recited in claim 138, wherein said sheath comprises asubstantially crystalline material.
 141. A nanowire as recited in claim140, wherein said substantially crystalline material is substantiallymonocrystalline.
 142. A nanowire as recited in claim 114: wherein saidnanowire is a functional component of a device selected from the groupof devices consisting essentially of phonon band gap devices, quantumdot devices, thermoelectric devices, photonic devices,nanoelectromechanical actuators, nanoelectromechanical sensors),field-effect transistors, infrared detectors, resonant tunneling diodes,single electron transistors, infrared detectors, magnetic sensors, lightemitting devices, optical modulators, optical detectors, opticalwaveguides, optical couplers, optical switches, and lasers.
 143. Ananowire as recited in claim 114, wherein said nanowire is an element ofan array of nanowires.
 144. A method of fabricating a nanowire,comprising: dissolving a first gas reactant in a catalytic liquidfollowed by growth of a first segment; and dissolving a second gasreactant in said catalytic liquid followed by growth of a secondcompositionally different segment joined to said first segment; whereinat least one of said segments has a substantially uniform diameter ofless than approximately 200 nm.
 145. A method as recited in claim 144:wherein a compositionally dissimilar liquid alloy is formed from eachsaid gas reactant and said catalytic liquid; and wherein each saidsegment forms upon saturation of said liquid alloy with a species ofsaid corresponding gas reactant.
 146. A method as recited in claim 144:wherein said first and second gas reactants comprise vapors generated bylaser ablation of a first and second growth species respectively.
 147. Amethod of claim 146, wherein said first and second gas reactants furthercomprise a carrier gas.
 148. A method as recited in claim 144: whereinsaid second gas reactant comprises a vapor generated by laser ablationof a growth species; and wherein said second segment comprises acombination of said species in said first and second gas reactants. 149.A method as recited in 144, wherein said catalytic liquid is formed froma preformed metal colloid.
 150. A method as recited in 149, wherein saidmetal colloid is part of a population of metal colloids with asubstantially monodisperse distribution of diameters.
 151. A method offabricating a nanowire, comprising: dissolving a gas reactant in acatalytic liquid followed by growth of a first segment; and coating saidfirst segment with a compositionally different second material andforming a second segment; wherein at least one of said segments has asubstantially uniform diameter of less than approximately 200 nm.
 152. Amethod as recited in claim 151: wherein a liquid alloy is formed fromsaid gas reactant and said catalytic liquid; and wherein said firstsegment forms upon saturation of said liquid alloy with a species ofsaid gas reactant.
 153. A method as recited in 151, wherein saidcatalytic liquid is formed from a preformed metal colloid.
 154. A methodas recited in 153, wherein said metal colloid is part of a population ofmetal colloids with a substantially monodisperse distribution ofdiameters.
 155. A method of fabricating a nanowire, comprising: forminga first segment by dissolving a first gas reactant in a catalytic liquidfollowed by growth of a first material; forming a second segment joinedto said first segment by dissolving a second gas reactant in saidcatalytic liquid followed by growth of a second material joined to saidfirst material; wherein each said segment forms upon saturation of saidliquid alloy with a species of said corresponding gas reactant; andcoating at least a portion of at least one of said segments with a thirdmaterial to form a third segment; wherein at least two of said materialsare compositionally different; and wherein at least one of said segmentshas a substantially uniform diameter of less than approximately 200 nm.156. A method of fabricating a nanowire, comprising: dissolving a firstgas reactant in a catalytic liquid followed by growth of a first segmentof material; dissolving a second gas reactant in said catalytic liquidfollowed by growth of a second segment of material joined to said firstsegment; and dissolving a third gas reactant in said catalytic liquidfollowed by growth of a third segment of material joined to said secondsegment; wherein, said first, second and third segments arelongitudinally adjacent; wherein said second segment is positionedbetween said first and third segments; wherein at least two of saidsegments comprise compositionally different materials; and wherein atleast one of said segments has a substantially uniform diameter of lessthan approximately 200 nm.
 157. A method as recited in claim 156:wherein at least two of said gas reactants are the same; and wherein atleast two of said segments comprise the same material.
 158. A method asrecited in claim 156: wherein a liquid alloy is formed from each saidgas reactant and said catalytic liquid; and wherein each said nanowiresegment forms upon saturation of said liquid alloy with a species ofsaid corresponding gas reactant.
 159. A method as recited in claim 156:wherein at least one of said gas reactants comprises a vapor generatedby laser ablation of a growth species; and wherein at least one of saidnanowire segments comprises a combination of species in said lasergenerated vapor and at least one other gas reactant.
 160. A method offabricating a nanowire heterostructure, comprising: dissolving a firstgas reactant in a catalytic liquid followed by growth of a first segmentof a first material; and dissolving a second gas reactant in saidcatalytic liquid followed by growth of a second segment ofcompositionally different second material longitudinally adjacent tosaid first material; wherein said second gas reactant comprises a vaporgenerated by laser ablation of a growth species; wherein acompositionally dissimilar liquid alloy is formed from each said gasreactant and said catalytic liquid; and wherein each said segment formsupon saturation of said liquid alloy with a species of saidcorresponding gas reactant; wherein said second material comprises acombination of said species in said first and second gas reactants; andwherein at lest one of said segments has a substantially uniformdiameter of less than approximately 200 nm.
 161. A method of fabricatinga nanowire, comprising: dissolving a first gas reactant in a catalyticliquid followed by growth of a first segment of a first material;sequentially laser ablating a growth species in the presence of saidfirst gas reactant thereby forming a second gas reactant; dissolvingsaid second gas reactant in said catalytic liquid followed by growth ofa second segment of a compositionally different second materiallongitudinally adjacent to said first material; wherein said secondmaterial comprises a combination of species in said first and second gasreactants; wherein at least one of said segments has a substantiallyuniform diameter of less than approximately 200 nm.
 162. A method asrecited in claim 161: wherein a compositionally dissimilar liquid alloyis formed from each said gas reactant and said catalytic liquid; andwherein each said segment forms upon saturation of said liquid alloywith a species of said corresponding gas reactant.
 163. A method offabricating a doped semiconductor superlattice nanowire, comprising:introducing a gas reactant into a reaction chamber of a furnacecontaining a substrate coated with a reactant metal; heating saidreaction chamber to a temperature at which said metal on said substrateliquefies into at least one droplet; dissolving said gas reactant intosaid liquid droplet until saturation where nucleation and growth of afirst segment; and dissolving a dopant and said gas reactant into saidliquid droplet until saturation wherein nucleation and growth of a dopedsecond segment occurs on said first segment; wherein at least one ofsaid segments has a substantially uniform diameter of less thanapproximately 200 nm.
 164. A method as recited in claim 163, whereinsaid substrate comprises an element selected from the group of elementsconsisting essentially of group III and group IV elements.
 165. A methodas recited in claim 163, wherein said metal comprises gold.
 166. Amethod as recited in claim 165, wherein said gold comprises colloidalgold.
 167. A method as recited in claim 163: wherein said substratecomprises silicon; and wherein said metal comprises gold.
 168. A methodas recited in claim 163, wherein said furnace comprises a quartz furnacereaction tube.
 169. A method as recited in claim 163, wherein said gasreactant comprises a mixture of mixture of H₂ and SiCl₄.
 170. A methodof fabricating an Si/SiGe superlattice nanowire heterostructure,comprising: depositing Au on a substrate; placing said substrate insidea quartz furnace reaction tube; introducing a gas reactant mixture of H₂and SiCl₄ into said reaction tube; heating said reaction tube to atemperature at which said Au liquefies into at least one nanosizeddroplet of an Au—Si alloy; and dissolving said gas reactant into saidliquid droplet until saturation where nucleation and growth of a Sisegment occurs; during said Si growth process, generating a Ge vaporthrough ablation of a Ge target with a laser; depositing both Ge and Sispecies into said Au—Si alloy droplets until saturation whereinnucleation and growth of a SiGe segment occurs on said Si segment;wherein at least one of said segments has a substantially uniformdiameter of less than approximately 200 nm.
 171. A method as recited inclaim 170, further comprising: pulsing said laser on and off; wherein aSi/SiGe superlattice is formed in a block-by-block fashion.
 172. Amethod as recited in claim 170, wherein said substrate comprises anelement selected from the group of elements consisting essentially ofgroup III and group IV elements.
 173. A method as recited in claim 170,wherein said gold comprises colloidal gold.
 174. A method as recited inclaim 170, wherein said substrate comprises silicon.
 175. A method asrecited in claim 144, 151, 155, 156, 160, 161, 163 or 170, wherein thediameter of said at least one of said segments having a diameter of lessthan approximately 200 nm does not vary by more than approximately 10%over the length of said segment.
 176. A method as recited in claim 144,151, 155, 156, 160, 161, 163 or 170, wherein said nanowire transitionsfrom said first segment to said second segment over a distance rangingfrom approximately one atomic layer to approximately 20 nm.
 177. Amethod as recited in claim 176, wherein transition from said firstsegment to said second segment begins at a point toward said secondsegment where the composition of said first segment has decreased toapproximately 99% of th e composition of said first segment at thecenter of said first segment.
 178. A method as recited in claim 144,151, 155, 156, 160, 161, 163 or 170, wherein at least one of saidsegments comprises a substantially crystalline material.
 179. A methodas recited in claim 178, wherein said substantially crystalline materialis substantially monocrystalline.
 180. A method as recited in claim 144,151, 155, 156, 160, 161, 163 or 170, wherein at least one of saidsegments comprises a semiconductor material.
 181. A method as recited inclaim 144, 151, 155, 156, 160, 161, 163 or 170, further comprisingdoping at least one of said segments.
 182. A method as recited in claim144, 151, 155, 156, 160, 161, 163 or 170, wherein said at least one ofsaid segments having a diameter of less than approximately 200 nm has adiameter ranging from approximately 5 nm to approximately 50 nm.
 183. Amethod as recited in claim 144, 151, 155, 156, 160, 161, 163 or 170,wherein said second segment is longitudinally adjacent said firstsegment.
 184. A method as recited in claim 144, 151, 155, 156, 160, 161,163 or 170, wherein said second segment is coaxially adjacent said firstsegment.
 185. A method as recited in claim 144, 151, 155, 156, 160, 161,163 or 170, further comprising doping said first and second segments toform a p-n junction.
 186. A method as recited in claim 185, wherein saidnanowire comprises a semiconductor device.
 187. A method as recited inclaim 144, 151, 155, 156, 160, 161, 163 or 170, further comprisingdoping a said one of said segments to form a p-i junction.
 188. A methodas recited in claim 187, wherein said nanowire comprises a semiconductordevice.
 189. A method as recited in claim 144, 151, 155, 156, 160, 161,163 or 170, further comprising doping a said one of said segments toform a i-n junction.
 190. A method as recited in claim 189, wherein saidnanowire comprises a semiconductor device.
 191. A method as recited inclaim 144, 151, 155, 156, 160, 161, 163 or 170, further comprisingelectrically coupling an electrode to at least one of said segments.192. A method as recited in claim 144, 151, 155, 156, 160, 161, 163 or170, wherein at least one of said segments comprises a material selectedfrom the group of elements consisting essentially of group II, groupIII, group IV, group V, and group VI elements, and tertiaries andquaternaries thereof.
 193. A method as recited in claim 144, 151, 155,156, 160, 161, 163 or 170, further comprising embedding at least one ofsaid segments in a polymer matrix.
 194. A method as recited in claim144, 151, 155, 156, 160, 161, 163 or 170, further comprising depositinga sheath over a portion of at least one of said segments.
 195. A methodas recited in claim 194, wherein said sheath comprises an amorphousmaterial.
 196. A method as recited in claim 194, wherein said sheathcomprises a substantially crystalline material.
 197. A method as recitedin claim 196, wherein said substantially crystalline material issubstantially monocrystalline.
 198. A method as recited in claim 144,151, 155, 156, 160, 161, 163 or 170: wherein said nanowire is afunctional component of a device selected from the group of devicesconsisting essentially of phonon bandgap devices, quantum dot devices,thermoelectric devices, photonic devices, nanoelectromechanicalactuators, nanoelectromechanical sensors), field-effect transistors,infrared detectors, resonant tunneling diodes, single electrontransistors, infrared detectors, magnetic sensors, light emittingdevices, optical modulators, optical detectors, optical waveguides,optical couplers, optical switches, and lasers.
 199. A method as recitedin claim 144, 151, 155, 156, 160, 161, 163 or 170, wherein said nanowireis an element of an array of nanowires.
 200. A laser, comprising: ananowire having a substantially uniform diameter of less thanapproximately 200 nm; and a pumping source.
 201. A laser as recited inclaim 200, wherein said nanowire comprises a plurality of segments ofcompositionally different materials.
 202. A laser as recited in claim200, wherein said pumping source is configured for exciting a populationinversion in said nanowire.
 203. A laser as recited in claim 200,additionally comprising a support material; wherein said supportmaterial is selected from the group consisting essentially of a solidsupport material, a liquid support material, a polymer support material,a glassy support material, and a substrate material.
 204. A laser asrecited in claim 200, further comprising a laser cavity.
 205. A laser asrecited in claim 204, wherein said cavity is contained within saidnanowire.
 206. A laser as recited in claim 204, wherein said nanowirehas ends that function as reflectors in said cavity.
 207. A laser asrecited in claim 200, wherein said pumping source is selected from thegroup consisting essentially of an optical source, an electrical source,a thermal source, an energy transfer source, a plasma source, a laser,and a flash-lamp.
 208. A laser as recited in claim 200: wherein saidnanowire comprises a coaxial heterostructure nanowire having a core andsheath; and wherein said pumping source is an electrical source in whichcurrent flows between said core and said sheath.
 209. A laser as recitedin claim 208, wherein said coaxial heterostructure nanowire represents ap-n junction.
 210. A laser as recited in claim 208, wherein anelectrical contact is made to said core and an electrical contact ismade to said sheath.
 211. A laser as recited in claim 208: wherein saidnanowire comprises a longitudinal heterostructure nanowire; and whereinsaid pumping source is an electrical source in which current flowsbetween one segments of said longitudinal heterostructure nanowire. 212.A laser as recited in claim 211, wherein said longitudinalheterostructure nanowire represents a p-n junction.
 213. A laser,comprising: a plurality of longitudinally adjacent segments ofcompositionally different materials forming a nanowire; at least one ofsaid segments having a substantially uniform diameter of less thanapproximately 200 nm; and a pumping source.
 214. A nanolaser as recitedin claim 213, wherein said pumping source is configured for exciting apopulation inversion in nanowire.
 215. A laser, comprising: a nanowirewith substantially faceted ends with a flat face oriented substantiallynormal to the longitudinal growth axis of said nanowire, having asubstantially uniform diameter of less than approximately 200 nm; and apumping source.
 216. A laser, comprising: a plurality of longitudinallyadjacent segments of compositionally different materials forming ananowire; at least one of said segments having a substantially uniformdiameter of less than approximately 200 nm; and a pumping source forexciting a population inversion in said nanowire.
 217. A laser,comprising: a nanowire having a substantially uniform diameter of lessthan approximately 200 nm; and a pumping source; wherein emission fromsaid laser is directed away from said nanowire in a direction parallelto the longitudinal axis of said nanowire.
 218. A laser as recited inclaim 217, wherein said nanowire is an element in an array of nanowires.219. A laser as recited in claim 218: wherein said nanowires in saidarray are aligned in substantially the same direction; and wherein laseremission from said array is directed in a direction substantiallyparallel to said wires in said array.
 220. A laser, comprising: ananowire having a substantially uniform diameter of less thanapproximately 200 nm; a plurality of quantum dots disposed in saidnanowire; and a pumping source.
 221. A laser as recited in claim 220,wherein said nanowire comprises a plurality of segments ofcompositionally different materials.
 222. A laser as recited in claim220, wherein said pumping source is configured for exciting a populationinversion in said quantum dots.
 223. A laser, comprising: a plurality oflongitudinally adjacent segments of compositionally different materialsforming a nanowire; at least one of said segments having a substantiallyuniform diameter of less than approximately 200 nm; a plurality ofquantum dots disposed in said nanowire; and a pumping source.
 224. Alaser as recited in claim 223, wherein said pumping source is configuredfor exciting a population inversion in said quantum dots.
 225. A laser,comprising: a plurality of longitudinally adjacent segments ofcompositionally different materials forming a nanowire; at least one ofsaid segments having a substantially uniform diameter of less thanapproximately 200 nm; a plurality of quantum dots disposed in saidnanowire; and a pumping source for exciting a population inversion insaid quantum dots.
 226. A laser as recited in claim 200, 213, 215, 216,217, 220, 223 or 225, wherein said nanowire comprises a substantiallycrystalline material.
 227. A laser as recited in claim 200, 213, 215,216, 217, 220, 223 or 225, wherein said substantially crystallinematerial is substantially monocrystalline.
 228. A laser as recited inclaim 200, 213, 216, 217, 220, 223 or 225, wherein said nanowire has adiameter ranging from approximately 5 nm to approximately 50 nm.
 229. Alaser as recited in claim 200, 213, 215, 216, 217, 220, 223 or 225,wherein the diameter of said nanowire does not vary by more thanapproximately 10% over the length of said nanowire.
 230. A laser asrecited in claim 200, 213, 215, 216, 217, 220, 223 or 225, wherein saidnanowire comprises a material selected from the group of elementsconsisting essentially of group II, group III, group IV, group V, andgroup VI elements, and tertiaries and quaternaries thereof.
 231. A laseras recited in claim 200, 213, 215, 216, 217, 220, 223 or 225, whereinsaid nanowire is embedded in a polymer matrix.
 232. A laser as recitedin claim 200, 213, 215, 216, 217, 220, 223 or 225, wherein said nanowireis an element of an array of nanowires.
 233. A laser as recited in claim238, 251, 253, 254, 255, 258, 261 or 263, wherein said pumping sourcecomprises an optical pumping source.
 234. A laser as recited in claim233, wherein said optical pumping source comprises an pumping laser.235. A laser as recited in claim 200, 213, 215, 216, 217, 220, 223 or225, wherein said pumping source comprises an electrical pumping source.236. A laser as recited in claim 235, wherein said electrical pumpingsource comprises an anode and a cathode.
 237. A laser as recited inclaim 236, wherein said anode is electrically connected to saidnanowire.
 238. A laser as recited in claim 237, wherein said electricalconnection comprises an ohmic contact.
 239. A laser as recited in claim237, wherein said electrical connection comprises a direct contact. 240.A laser as recited in claim 236, wherein said cathode is electricallyconnected to said nanowire.
 241. A laser as recited in claim 240,wherein said electrical connection comprises an ohmic contact.
 242. Alaser as recited in claim 240, wherein said electrical connectioncomprises a direct contact.
 243. A laser as recited in claim 236,wherein said anode and said cathode are electrically connected to saidnanowire.
 244. A laser as recited in claim 243, wherein said electricalconnection comprises an ohmic contact.
 245. A laser as recited in claim243, wherein said electrical connection comprises a direct contact. 246.A laser as recited in claim 200, 213, 215, 216, 217, 220, 223 or 225:wherein said nanowire has first and second ends; and wherein said firstand second ends have reflective surfaces.
 247. A laser as recited inclaim 246, wherein said nanowire comprises a cavity.
 248. A laser,comprising: a multi-faceted, single-crystalline, ZnO nanostructurehaving a substantially uniform diameter of less than approximately 200nm; said nanostructure having first and second ends; said first endcomprising an epitaxial interface between said nanostructure and asapphire substrate from which said nanostructure extends; said first andsecond ends having corresponding reflective faces; wherein saidnanostructure functions as a resonant cavity between said end faces.249. A laser as recited in claim 248, wherein said nanostructure isembedded in a polymer matrix.
 250. A laser as recited in claim 248,wherein said nanostructure is an element of an array of nanostructures.251. A laser as recited in claim 248, further comprising a pumpingsource.
 252. A laser as recited in claim 251, wherein said pumpingsource comprises an optical pumping source.
 253. A laser as recited inclaim 252, wherein said optical pumping source comprises an pumpinglaser.
 254. A laser as recited in claim 251, wherein said pumping sourcecomprises an electrical pumping source.
 255. A laser as recited in claim254, wherein said electrical pumping source comprises an anode and acathode.
 256. A laser as recited in claim 255, wherein said anode iselectrically connected to said nanostructure.
 257. A laser as recited inclaim 255, wherein said cathode is electrically connected to saidnanostructure.
 258. A laser as recited in claim 255, wherein said anodeand said cathode are electrically connected to said nanostructure. 259.A laser as recited in claim 256, 257 or 258, wherein said electricalconnection comprises an ohmic contact.
 260. A laser as recited in claim256, 267 or 258, wherein said electrical connection comprises a directcontact.
 261. A laser cavity, comprising: a semiconductor structurecapable of exhibiting quantum confinement effects; wherein saidsemiconductor structure comprises a laser cavity.
 262. A laser cavity asrecited in claim 261, wherein said semiconductor structure comprises ananowire.